The terms tend to be reasonably well-defined within a field of study, but there's essentially no coordination between fields, at least not in the Anglosphere.

For example, in historical linguistics, Early Modern English is the period of the language from the start of the Tudor dynasty to the Restoration, when the transition to the modern form of the language began. Middle English, before the Tudors, is fairly difficult for speakers of Modern English to read. Early Modern English, such as the original language of the King James Bible and of Shakespeare, is quite comprehensible, although the spelling is often modernised nowadays. True Modern English was written from the mid-eighteenth century onwards and presents no problem, although it's often possible to tell approximately when something was written via changing fashions in word use and idioms.

So Modern English was formed before the end of the OP's Early Modern period, and there's no "post-modern" period in linguistics; as others have said, that's a cultural term, rather than a historical one.

Here is a chronological explanation for the above mentioned (Western) historical periods:

1. Early Modern History: Begins with The Northern Italian Renaissance around 1400 AD/CE. The Early Modern period also includes, the rise of Iberia-(Spain & Portugal) during the 1500's, followed by the 1600's-(the rise of the Dutch Empire & Elizabethan England).

2. Late Modern History: Begins with the 1700's, specifically, with the rise of the Enlightenment in France, followed by the emergence of England as a major financial and technological power, through The Industrial Revolution, the Victorian Age, whereby much of British colonialism expanded during this time under Queen Victoria, as well as The Romantic Age, which produced great Poets, such as Byron, Wordsworth & Keats. The Late Modern Age also produced some of the most significant Writers in History, such as Dickens, Marx and Darwin. And of course, much of the 20th century is also part of the Late Modern Age, ending with the collapse/implosion of Global Communism and its Epicenter, The Soviet Union in 1991.

3. Post-Modern History: This is the Contemporary historical period, beginning in the immediate aftermath of the collapse of the Soviet Union. We have been living in the Post-Modern period since 1992, that is to say, a Post-Soviet/Post-Communist (and even Post-Industrial) era, which helped to spawn The Age of Information, as well as The Age of Globalization. The Information Age has and continues to "deconstruct" Modern conventionality through our modes of communication and thought and The Globalization Age has transformed the movement of capital and business into worldwide enterprises and industries. Both Information and Globalization have a symbiotic relationship whereby one necessitates the other. The Information age is inherently Global and the Globalization Age is enhanced by the worldwide access to Information.

Periods in these terms normally pertain to the arts (philosophy,drama,fine art, etc.) and how they are effected by all disciplines and fields of thought. Ideas that are invented during a period, reach a level of acceptance, and are incorporated into new thought as the decades pass, can help define a new period that follows by the fact that it becomes sewn into the work of the new/ensuing period.

For example, Friedrich Nietzsche's philosophy - let's take his work on the aeshetic ideal, became incorporated into thought during the modernism period, and then emerged from literature in a new period, called post-modernism. It can be seen in 60 and 70s literature, and thereafter. Same with Einstein, who's thoughts on space/time, and Cezanne, who's paintings experimented with these ideas, was carried forward into Pablo Picasso's work through modern and into post-modernism.

This is a summary or perspective on how periods work. The more you studies these areas, the clearer it will be.

In dutch it's a lot easier: 1500-1800 is the nieuwe tijd, wich translates to new time, and 1800 to now is the hedendaagse periode, wich translates to contemporary period. I've never heard of a distinction for the period after world war II.

I can maybe help you with the first question though: we define the start of the early modern period by events like the fall of Byzantium in 1453, the invention of the printing press by Gutenberg in the same period and the discovery of America by Columbus. The end is mainly defined by the french and the American revolution, and the fall of Napoleon.

I have heard of the postmodern period before but never in history classes, just behavior sciences (in high school). I just did a quick google and it seems almost all results are about literature, architecture and culture in general. I think it is rarely used in history.

Apples & oranges.

I don't think there are official, or even conventional definitions for any of these terms; they vary depending on context. If you're talking to a paleontologist, the definition of the modern era will be very different from the definitions used by a historian specializing in "Democracy" or "women's rights", or whatever.

"Postmodern" isn't - at least in my opinion - a term with a solid meaning in history. Postmodernism has a meaning in art and in philosophy, but not AFAIK in history. A friend of mine has suggested the postmodern drinking game where every player attempts to define "postmodern". Everyone takes a drink unless any two definitions match. He proposes this because it is the only way of terminating any discussion about what postmodern means. (and the process of terminating the discussion is far more enjoyable than the discussion) The best definition I've ever read is in Perl the first postmodern programming language

Rule of thumb?

• Pre-modern is the belief that I'm right and you're wrong.

• Modern is the belief that I'm right and that when I've explained myself to you sufficiently, you'll realize that you agree with me.

• Postmodern is the belief that we're both right, and that if we spend enough time talking about that, we'll never get anything done, so the best course of action is to ignore the problem. (and according to the rules above, unless you agree with me, we're both obliged to drink a shot now.)