Can you tell me which word, lay or lie, should be used? Do you have some general tips?

His interests lay in developing versatile resource book online.
To lie is to tell something that is not true.

To lay is to be ready for sleep (lay on the bed).
Sorry, Tyx. It's to lie on the bed or to lay something on the bed. Emotion: smile
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lie, lied, lied: tell an untruth.

Aside from lie in the meaning above, there are two verbs of similar meaning: lie and lay.

lie is intransitive. lie, lay, lain (You don't lie something. You just lie somewhere.)
lay is transitive. lay, laid, laid (You lay something somewhere. You don't just lay somewhere.)

The point of confusion is that lay is the past of the intransitive verb AND the present of the transitive verb.
(In your example lay is the past of the intransitive verb: His interests lay in ....)

I lie on the bed when I am tired.
He lay on the grass until he recovered from the shock.
She has lain on the sofa all day. I think she is sick.

I am going to lay this money on the table.
Mary laid the child down in the crib.
Sam has laid the letters on the table.

Hello CalifJim,

I must admit that I also have problems with these verbs!:)

..and what about the gerund? "Lying" for the intransitive verb?

Thanks in advance!
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Aha, I found it what I wanted to know, thanks Marius!:)
..I always forget to search for other threads..