Grammar: lay or lie?
The CD-ROM version of the OED gives a clear definition of lay, "to cause to lie", and lie means "horizontal or resting position". So, a hen lays an egg, and the eggs lie in the straw. You lie on a yoga mat and you lay down to do so.
To use the word lay you mean place something down carefully, to put something down flat or to sleep with someone. The easiest way to remember this is the term 'lay the table' or 'the hen lays an egg'. The word lie is used to refer to something that is already down, as in "I just want to lie down".
Where it gets confusing is in verb forms, which goes Lay > Laid > Laid and Lie > Lay > Lain. Lay' can be the past tense of Lie or the nominative (normal dictionary form) of Lay, hence the confusion.
About the author
Jesse Karjalainen is the author of The Joy of English: 100 conversations about the English Language, Cannibal – the language and history of the discovery of the New World, and Roanoke – the language and history of Early Colonial America.