when to use "lay" and when to use "lie" ? I know "lay" is the past tense of "lie", but then why people say "pls lay the book down" not "pls lie the book down" ? pls lay the book down = pls lie down the book??
We usually say, " I am going to lie down"., but why people use "lay" in " Please lay the book down" ? Please clairfy. thanks.
This is a common problem for English speakers, not just for learners. The problem is that forms of the two verbs overlap. The two verbs are:

To lay, laid, laid. - to place something on something, usually the floor. (Transitive)
To lie, lay, lain. - to place oneself into a recumbant position, often on a bed, or a sofa. (Intransitive)

Learn those six words off by heart.

To lay down - to place something at a lower position.
To lie down - to place oneself in a lower recumbant position

My advice is to keep the down close to the lie or lay. So your second example would be better put: please lay down the book.
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This is not difficult.

Lay (lay, laid, laid) is transitive. It requires an object: I lay my toothbrush in the rack every night.
Lie (lie, lay, lain) is intransitive. It takes no object: I lay on my bed till late yesterday morning.
We've got two threads running simultaneously on this.
Thanks, Tom. I just Merged them. (Maybe it would have been better to leave them apart?)
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Thank you both. I think I know how to use them now.