I’ll never forget the day my mother (a former English teacher) had me tell my sixth-grade teacher she was using “lain” incorrectly. Instead of reinventing the wheel, here’s a little help from Writer’s Digest.
In the past tense, “lay” becomes “laid” (Last week, I laid down the law and told my brother it was inappropriate for him to pick his nose), and “lie” becomes “lay” (Yesterday he lay down for a nap that afternoon and picked his nose anyway). Yes, “lay” is also the past tense of “lie.” And the confusion doesn’t end there.
To throw you for another loop, “laid” is also the past participle form of “lay.” So, when helping verbs are involved, “lay” becomes “laid,” and “lie” becomes “lain.” Aunt Sally had laid the turkey in the oven earlier this morning. The chicken had lain there all day until it was cooked all the way through and ready for us to eat.
Remember: Lay and laid both mean to set something down, while lie, lay, and lain all mean the subject is setting itself down.