What’s the difference between lay and lie?
You lie down, but you lay something down. Lie does not require a direct object. Lay requires a direct object. The same rule applies to laying and lying (not lieing—beware of spelling). The past tense of lay is laid, but be careful with the past tense of lie—there are two options because "lie" has two meanings. Lie can mean to recline or it can mean to tell an untruth.
This mnemonic should help you remember that lay, which begins with the letters L-A, has a long A sound like its definition: to place. On the other hand, lie, which starts with the letters L-I, has a long I sound like its definition: to recline.
Another way to remember:
LIE and LAY work like SIT and SET.
I am going to go lie down. I am going to sit down.
I am going to lay the baby in the cradle. I am going to set the books on the table.