Yes,
When you lie down, you are horizontal.
When you lie down, you are probably needing to rest.
This is an idiomatic phrase, and the correct verb is "lie" (which you do) rather than lay (which you do to something else)."You really need to lie down" means "You should lie down."
you do not have to put the comma there
Yes, although it is somewhat idiomatic. It means you think that you should lie down.
The present tense spelling for "lie down" is "lie down."
it needs a comma
You should never lie to the one you love.I need to lie down.
It depends. Here are examples: In this sentence, a comma is only necessary after the "for example": For example, yesterday she fell down and didn't even cry. In this sentence, a comma is necessary before and after "for example": You, for example, would also be depressed if you were failing trigonometry.
No, it is not a preposition. Lie down is a statement made from a verb and an adverb.
you should lie down on the chair because if you lie down on the floor you can get dizzy
"In five years, things happened." Yes you do need a comma.
Yes, after rapid you need a comma and another action like: while they floated down the rapid, they ate sandwiches.
No. There is no word in English that always requires a comma before it.