What is the difference between lay and lie?

Lay needs an object — you lay something down. Lie does not — you lie down yourself.

I lay the book on the table. (the book is the object) I lie on the sofa. (no object)

That much is manageable. What makes this pair genuinely difficult is the past tenses, because the past tense of lie is lay — the same word as the present tense of the other verb.

| | Present | Past | Past participle | |---|---|---|---| | lay (place something) | lay | laid | laid | | lie (recline) | lie | lay | lain |

So:

Yesterday I lay on the sofa. (past of lie — correct, though it sounds wrong to many ears) Yesterday I laid the book down. (past of lay) The book has lain there for weeks. (past participle of lie)

The overlap at lay is the entire problem, and it is why this is one of the few usage distinctions that genuinely confuses careful writers rather than careless ones.

A separate verb, lie meaning to tell an untruth, is regular: lie, lied, lied. No confusion there.

The substitution test: try place in the sentence. If place works, you want lay. I place the book down ✓ so I lay the book down. I place on the sofa ✗ so it must be lie.

In speech the distinction is widely collapsing, and lay down for lie down is very common. In edited writing it is still expected.

The same pattern affects two related pairs. Sit and set work identically: you sit down yourself, you set something down. So do rise and raise: prices rise, you raise prices. In each pair the one with an object is the one people reach for wrongly.