Lay, lie, and the other lie seem designed to cause trouble. Two are homonyms, and the present tense of one is the same as the past tense of another.
Mix-ups usually come when we mistake the intransitive lie—she was lying on the floor—with the transitive lay. If she was laying on the floor, she needs an object—she was laying her yoga mat on the floor. But most of us usually use the simple past tense. She laid her yoga mat on the floor, and then she lay down on it. When asked her age, she lied.
Many of us have to look up the past participle every time: She had laid the mat on the floor. She had lain on it a long time. She had lied about her age for years.
One good way to ensure you have the right conjugation is to substitute a synonym. She reclined the mat on the floor would never get past you.
Here’s a chart to help keep things sorted:
|Definition||to put down||to recline||to tell an untruth|
|First person present tense||lay||lie||lie|
|Third person present tense||lays||lies||lies|
|Simple past tense||laid||lay||lied|
This chart is also available as a PDF: Lay Lie card