There are two words. Lie and lay, they're different verbs.

The difference is that "lie" is something that's done, and "lay" is something that's done to something else. It requires a direct object.

So you lie down. But you lay down the sheet.

Where it gets a little weirder is in the past tense.

The conjugation of "lie" is lie/lay/lain. The conjugation of "lay" is lay/laid/laid. As you can see, the tricky part is that the past tense of "lie" is "lay", but it means a different thing than the present tense "lay".

But it still works just the same as before. One is something you do, the other is something you do to something else.

So you lie down. Yesterday you lay down. Every day this week you have lain down.

You lay the sheet down. Yesterday you laid the sheet down. Every day this week you have laid the sheet down.

Hopefully that helps?