According to the Young Earth Creationism, what does it mean Genesis 1:14 and 18?

Short Answer

before the event of Gen 1:14, the day and the night are mixed, not separated yet. After that event, then they are separated.

Using Ken Ham's hypothesis, the day and the night were separated just after Gen 1:3.

What is the difference at the event of separation and the condition after each separation the light from the darkness event which occurred in Genesis 1:4 and then occurred again in Genesis 1:18

The difference in condition is in the source of the light: in Gen 1:4 it's after the creation of temporary light (Gen 1:3), but in Gen 1:18 it's after the creation of greater & lesser lights (Gen 1:14-17). But it's very important NOT to read the separation as an ACTION, but as a description of the EFFECT of "Let there be light" (v. 3) and "Let there be lights in the expanse of the heavens to separate the day from the night. ... And it was so." (v. 14-15) respectively. Using Ken Ham's hypothesis (see Long Answer), in both "separation" the meaning is the same. It is to contrast the lighted part of the globe from the total darkness (v. 4b) of the unlighted part of the globe (the Night, v. 5a).

To me, right after the light created, there is no darkness anywhere at all. The whole globe surface get the light created in verse 3. There is no a part of earth globe in light and another part in darkness.

We don't have to read it this way. The Bible allows for Ken Ham's hypothesis where in v.3 there was a light shining on earth in the manner like the current sun (see quote in the Long Answer).

what does it mean the act of God in verse 4b?

God DID NOT do anything in vv. 4-5a. All God did was the SINGLE ACTION to put the light in the sky in v. 3. The "separation" was the EFFECT, not an action.

does it mean that verse 5 can be put before verse 4b ? In other words, without the separation act of God in verse 4b, there is already "day" and "night" ? the light is already separated from the darkness before verse 4 ?

No. Verse 5b needs to wait until all parts of earth experienced Day and Night, so 24 hours needed to pass between verse 3 and verse 5b. Verses 4-5a is simply a DESCRIPTION of the effect of the SINGLE action in v. 3. With that single action, half the globe had Day (daylight brightness) and the other half had Night (total darkness). (See the picture in the Long Answer below.) Twelve hours later, the half that had Day now started to have Night. Therefore, 24 hours after God put the light in v. 3 every part of the globe had experienced an evening and a morning. Verse 5b is the description after that 24 hours.

Long Answer

Background

There are several ways to read Gen 1. You have asked for the modern Young Earth Creationist perspective, which reads Gen 1 as six 24-hour day creation week. YEC tends to use the following principles:

• Modern YEC reads the Bible as literal / natural as possible unless allegorical / metaphorical / figurative meanings are clearly intended (such as the book of Revelation). For example, a YEC Radioisotopes and the Age of the Earth (RATE) project determines that Genesis should be read as history.
• Modern YEC also reads the Bible using the historical-grammatical method to discover the authors' original intended meaning in the text. For example, if the author intends Genesis to be read as history, then YEC would reject a symbolic, mythical, historical-critical, allegorizing, or other kind of readings.
• There are different YEC interpretations of Gen 1. But in ALL YEC interpretations, when there is an apparent conflict between science and YEC's literal and natural reading of Gen 1 to Gen 11, YEC always gives priority to YEC's way of Bible reading.
• YEC distinguishes between empirical science (which they embrace) and historical science (which they vet carefully). When historical science is done with certain conjectures (such as antitheistic Methodological Naturalism), YEC deems the practitioners to overstep science's authority because the findings may exclude the possibility of God's miracles in the past (even if a miracle is the best explanation) and instead replace God with theories like evolution, old earth geology, etc. More explanation here.

Just for perspective, here are several examples of how non-YEC reads Genesis 1 using non-literal interpretations:

• consider the influence of Ancient Near East cosmology to the author of Genesis, who then conceived the heavens, the firmament, the oceans, and the earth to be shaped like this.
• consider a literary reading of Genesis 1 using the framework view in this BioLogos article How long are the days of Genesis 1?
• consider an example of reading light and darkness symbolically

But for this answer we will use the YEC principles above.

Ken Ham's YEC hypothesis

Ken Ham is CEO and co-founder of a premier Young Earth Creationist institution: Answers in Genesis (AiG).

For this answer we will go with Ken Ham's hypothesis described in a 2019 AiG blog article Light Before the Sun: How biblical apologists have historically understood the source of light before the sun was created in Genesis 1:

The first three days are written the same way as the next three. So if we let the language speak to us, all six days were ordinary earth days. . . . The sun is not needed for day and night. What is needed is light and a rotating earth. On the first day of creation, God made light (Genesis 1:3).

The phrase “evening and morning” certainly implies a rotating earth. Thus, if we have light from one direction, and a spinning earth, there can be day and night.

More background, explanation, and support for this hypothesis:

• For more details on Ken Ham's hypothesis as well as a historical survey of various YEC positions, see my answer to your other question about Gen 1:3.
• Ken Hamrick, a self-professed Augustinian realist and supporter of YEC views, wrote a very similar explanation in his 2015 blog article Shedding Light on the Length of Pre-Sun Creation Days: A Text-Based Approach.
• Dan Kreft, a YEC apologist, has a 1/2 hour video presentation How Does One Measure Days Before God Created the Sun? (2020) showing how having a 24-hour day and a rotating planet from day 1 is necessary, although the source of light is different from Ken Ham's hypothesis.

Commentary

We will then use Ken Ham's hypothesis and the YEC principles described above to interpret Gen 1:1, 1:3-5, and 1:14-18 verse by verse. We will use the ESV literal translation as well as checking the underlying Hebrew to make sure our translation doesn't add words that are not there in the original Hebrew text.

• Gen 1:1 "In the beginning, God created the heavens and the earth".

  • It means that the earth was created a globe and was already rotating at the current speed. There was also a sky.

• Gen 1:3-5 "³ And God said, 'Let there be light,' and there was light. ⁴ And God saw that the light was good. And God separated the light from darkness. ⁵ God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day."

  • In v. 3 "light" refers to temporary source of light provided by God before the creation of the sun. The "light" was placed in the sky in such a way that provided lighting to only a part of the earth globe at a given time, just like how the sun lights the earth only partially today. Imagine God putting the light to the LEFT (OUTSIDE) of the picture below: _{Photo credit: "The planet Earth rotating, half in shadow." by A Luna Blue}
  • The light was created instantaneously (within a split second). And as you say, "the creation of light itself 'automatically' causing a separation, some part is bright - some part is dark." With that single action, half the globe (the left side in the picture) had "day" and the other half (the right side in the picture) had "night".
  • God didn't do anything further in verses 4-5a. Verses 4-5a is simply DESCRIBING the effect of the SINGLE action in v. 3. In other words, as a consequence of putting the light to the left of the picture, God "separated" (as description of the effect) the area of the globe that received the "light" (daylight), calling it Day, from the area of the globe had "darkness" (no light at all), calling it Night.
  • Note that the "light" called Day in vv. 4b-5a refers to the daylight (the brightness of the day in the left side of the globe in the picture above). This is different than the light source which God created when He said "Let there be light" (not shown in the picture above, shining from the outside left of the picture). What God called "good" was the light source itself (vv. 3-4a), not the daylight (vv. 4b-5).
  • Note also that the "darkness" called Night in v. 5a (the total absence of light in right side of the globe in the picture above) is total darkness, since there was no moon or stars.
  • Twelve hours later, the half that had Day now started to have Night.
  • After another twelve hours, the earth had undergone a single full rotation. This was now 24 hours after God put the light in v. 3. By then, every part of the globe had experienced an evening and a morning. Since this is the first time that the WHOLE earth experienced both daylight and night, it's appropriate for this 24 hour period to be called "the first day". Thus v. 5b is simply DESCRIBING the effect 24 hours after God said "Let there be light."

• Gen 1:14-15 "¹⁴ And God said, 'Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years, ¹⁵ and let them be lights in the expanse of the heavens to give light upon the earth.” And it was so."

  • Notice how the phrasing "separate the day from the night" is different than "separated the light from the darkness" in Gen 1:4. The change was because of a change in situation, since in Day 4, the earth was now lighted with a different source, and the night in the 4th day was now no longer total darkness but had "lesser light". The word "separation" still makes sense though, since the day was still much brighter than the night as it is today. Again, like in v. 4b, "separation" here is an effect, not an action. The action in the 4th day was creation of the sun, the moon and the stars.
  • Notice also how the meaning of "light" in Gen 1:4b is the same with the "day" in Gen 1:14. Both refers to the daylight (the effect of the light rather than the source).

• Gen 1:16-18 "¹⁶ And God made the two great lights -- the greater light to rule the day and the lesser light to rule the night -- and the stars. ¹⁷ And God set them in the expanse of the heavens to give light on the earth, ¹⁸ to rule over the day and over the night, and to separate the light from the darkness. And God saw that it was good."

  • The "greater light" was obviously referring to the sun, either created from scratch, or from repurposing the temporary light in Gen 1:3. The "lesser light" was obviously the moon. See Matthew's answer.
  • What's called "good" in v. 18 was the whole arrangement of the present sun, moon, and stars giving light to rule the night and different light to rule the day, as well as their being "for signs and for seasons". Obviously, "good" in v. 18 is a different kind of goodness than the "good" in Gen 1:4a, which was referring to the goodness of the temporary light before the creation of the sun.
  • Notice how verse 18a says "separate the light from the darkness". There is a change BACK to verse 4 "separated the light from the darkness". I think this is to communicate a different idea than in verse 14b "separate the day from the night". This is to highlight the benefit of having some light during BOTH the day AND the night, compared to having no lights at all (the total darkness of the Night in verse 5a before the moons & stars). Again, "separate" here is EFFECT, not action. The action was in v. 16 ("made the two great lights") and v. 17 ("set them in the expanse of the heavens").