Did this dog lay down or lie down? Does anyone else in the world have trouble figuring that one out? 

When to use lay instead of lie, or vise-versa, is a sharp thorn in my writerly side. My misuse of this word is a pain in the behind for my critiquers as well. So this post is actually more for me than my readers, but maybe it will benefit you as well.

The thing that's so tricky about lay and lie is that the past tense of lay is laid, and past tense of lie is lay. Whoever came up with that brilliant idea ought to be boiled in oil.

For all the language geeks out there, here's the technical mechanics of how to use each word...

LAY 
This is a transitive verb, meaning it requires a subject and one or more direct objects.

LIE
This is an intransitive verb, meaning it does NOT need an object.

And just in case that opens up the ol' what's-an-object can of worms:

OBJECT
An object in grammar is part of a sentence and often part of the predicate. It donates somebody or something involved in the subject.

Basically, lay needs an object. Lie doesn't. Let's give this a whirl in present tense, which is the easiest. You LAY something down. People LIE down by themselves.
You lie down on the couch.
You lay the book down on the table. (the book is the direct object)

Easy peasy. It's that stinker of a past tense that throws me over a cliff. Here's a handy dandy little chart that I intend to print out and have tattooed on the back of my hand...




That helpful? How about this...
Lay, laid, laid, laying means to place (for instance, he lays a book on the table).
Lie, lay, lain, lying means to be in the horizontal position (it doesn't require an object).

Or how about this...
Try the "rest/place" test. Substitute a form of lie or lay with a form of rest or place. If a form of rest makes more sense, use a form of lie. If a form of place makes more sense, use a form of lay.
Rest (rested, resting) = lie (lay, lain, lying)
Place (placed, placing) = lay (laid, laid, laying)

So, what about that dog? The dog lies on the ground. And after this mind-twisting post, I think I'll go take a nap as well.