score:5
Before going on, note Article of Faith 8 (doesn't necessarily apply to this verse just keep in mind when getting a Mormon's interpretation of a Bible verse):
We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God.
From Psalms 110:1 (KJV on LDS site)
The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
Besides Matthew 22:41-45 this Psalm is also referenced in Acts 2:32-36 and Hebrews 1:13.
The LDS believe that LORD (uppercase) is used in place of printing Jehovah, who we believe to be Jesus Christ, during the translation process. 1,2
Lord (in lowercase) refers to a heavenly ruler but is not a substitute for the name Jehovah. 2
There are at least 3 instances when LORD applies to Heavenly Father, not Jesus Christ and this verse is one of them. 2 ftnt 3 So the verse is really saying:
The LORD [Heavenly Father] said unto my Lord [Adona/pre-incarnate Son], Sit thou at my right hand, until I make thine enemies thy footstool.
So in Matthew Jesus is showing that he understands 'the LORD' refers to Heavenly Father and 'my Lord' refers to the Son of God and Messiah. 3
Acts 2:36 also backs this up
Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
footnote 3:
In at least three Old Testament passages it appears that LORD applies to Heavenly Father, not Jesus Christ: Ps. 110:1; Ps. 2:7; Isa. 53:10
3 Names of God in OT article- highly recommend for a much better in-depth explanation, you can start half way down where it talks specifically about Psalm 110
Abinadi on the Father and Son mentions the Acts2:36 reference to the Psalm