Do people “lie down”, “lay down”, or both?
Sheesh, I have googled this zillions of times. And read carefully and “commit it to memory” and then the next time it comes time to use one, I can not remember.
http://wvde.state.wv.us/tt/2002/grammartips/grammartip0014.html
Here’s a link explaining the answer, hopefully it’ll help you, I know I won’t remember. 
Here’s another link: http://grammar.quickanddirtytips.com/lay-versus-lie.aspx
You lie down, but you lay something down. Lay takes an object. It’s a transitive verb. Lie doesn’t take an object; it’s an intransitive verb.
The confusing part is that the past tense of lie is lay - a different lay.
Present…Past…Past Participle
Lay…Laid…Had Laid
Lie…Lay…Had Lain
If you’re asking what’s proper English: people lie down (as in “I’m tired. I’m going to go lie down.”)
If you’re asking what people actually commonly say, it’s both. “I’m going to go lay down” is incorrect, but a lot of people say it.
The easiest way I know of to explain it is that:
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They’re both irregular verbs: “I lay/laid/have laid…” vs. “I lie/lay/have lain…”
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“Lay” is a transitive verb; it demands an object. You lay something somewhere. It’s synonymous with “to place”.
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“Lie” is intransitive, and synonymous with “to recline.”
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Both verbs also have a second meaning: lay for “have sexual relations with” and lie for “prevaricate, tell a falsehood” (which is semi-regular: “I lie/lied/have lied…”)
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As noted, “lay” is also the past tense of “lie” – a distinct usage from the “place” meaning.
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Two points need making: In traditional English usage, a valid periphrastic constructin for “to lie” is “to lay oneself [down]” . For bonnie Annie Laurie, the singer tells us he would “lay [me] doon and dee.” And, probably based on this, informal colloquial speech accepts them as synonyms with the meaning “recline” (though not for “place”). Note that that’s not “wrong” – it’s a usage in a particular register that is not acceptable in a formal written register.
Would language experts call this a second meaning of the same word, or a separate word (with its own, separate entry in the dictionary)?
Does the mean that I lie when I said that I lay down when I get laid?
When is the last time you heard the word “lain”? I don’t remember ever hearing it other than in grammar class (or its homophone “lane” of course).
They are generally listed separately, as “lie[sup]1[/sup]” and “lie[sup]2[/sup]”; they have different roots, though both are Germanic.
FWIW, I use the beginning of the children’s prayer “Now I *lay me *down to sleep” as a mnemonic to help me recall that lay is the transitive version and lie is the intrasitive version. Thus, “I am going to lay down this book,” but “I am going to lie down and have a nap,” because the first takes a direct object but the second does not.
I like to remember that people are not eggs. That is, it’s easy to remember that chickens lay eggs. The one that people do is the other one - lie.
What makes this other verb “lie” only semi-regular? It seems completely regular to me, in all the present tense and past tense forms.
Or does its present participle, “lying”, mark it as semi-regular? In which case the same would apply to “die”, “tie”, and “vie”.