# Now then, in the sight of all Israel ... all of you must

Here David is giving a command to all of the people of Israel. Alternate translation: "Now then, in the sight of all Israel, this assembly for Yahweh, and in the presence of our God, I command all of you to" 

# in the sight of

"in the hearing of" 

# keep and try to carry out

"carefully obey"

# your descendants after you forever

"your children and your descendants, who will take your place after you die"