What's the difference between "lay" and "lie"?

In my lessons, I sometimes write something like this:

Your son usually sleeps in the afternoon after lunch. You want him to lay down and start to fall asleep.

When I do, I get questions like this one:

"You want him to lay down" or "You want him to lie down"? I am confused.

So how should you use "lay" and "lie"?

The correct use of "lay" and "lie" 

"Lay" and "lie" are confusing and even controversial, both for English learners and for native English speakers.

Strictly speaking, "lay" is a transitive verb. In other words, you "lay" something somewhere:

When I get home I usually lay my keys on the table next to the front door.

I think you laid it on the table.

We're supposed to use "lie" intransitively. In other words, it does not have an object. We use it like this:

Why don't you lie down?

Sometimes I like to lie in the grass and look up at the sky and think.

But there's an extra complication: when you're talking about the past, you're supposed to use "lay":

Clark wasn't feeling, so I lay down with him until he wen to sleep.

And another complication: we use "have lain":

I have never lain on a more comfortable bed!

How English speakers use "lay" and "lie" in practice

The rules outlined above are complicated. Some English speakers follow them. But most don't. That might be because:

• they don't know the rules for "lay" and "lie"
• they know the rules but don't follow them
• they know the rules but only follow them in formal situations

So how do most English speakers use "lay" and "lie"? We say "lay" for present tense and "laid" for past tense:

When I get home I usually lay my keys on the table next to the front door.

Why don't you lay down?

I think you laid it on the table.

Clark wasn't feeling, so I laid down with him until he wen to sleep.

And we use "have laid":

I've never laid on a more comfortable bed!

Which rules should you use?

It's up to you.

A person who uses "textbook" grammar at all times might seem polished, professional, and sophisticated. But they might also seem cold or snobby.

A person who follows popular usage might seem warmer and friendlier. But in some situations, others might see them as less intelligent.