Here are the main points to remember:

• Lay means "to place or put something down" and it needs an object. For example, "She lays the book on the table."
• Lie means "to be in or assume a flat position" and it does not need an object. For example, "He lies on the couch." It can also mean "to say something that is not true".
• The past tense and past participle forms of lay and lie are irregular and sometimes overlap. For example, the past tense of lie (to recline) is lay, which is also the present tense of lay (to place). ^{1 2}