Lie vs lay

The first is standard English (also used by educated Americans when they want to) and the second is American slang.

Second answer:" To lie" is something you do yourself, or ask someone else to do. "To lay" is something you do with an object.

Examples.
I think I'll lie down here for a nap.
Lie here, if you're tired, and I'll get you on the way back.
I'll lay this fresh tablecloth on the table before dinner.

A dictionary will help with all the other meanings of yhe word "lie". And with a few other meanings of "lay", too.

Thank you McTag. It was very useful. I will follow your advice. Australians use both and it doesn't make everything easy. I wrote an essay and my teacher said that I should use "lie" instead of "lay" in the sentence I used. You helped me understand why.

You're welcome. Glad to help.
Pop songs don't help, they almost always opt for "lay" in that context:

"Come and lay down by my side (Help me Make it Through the Night)*
"Lay lady, lay
Lay across my big brass bed"

....and so forth. But it's important to know the difference.

I lay down my gun, and just shook my head at McT.