How do I know when to use lay or lie?

Question

Thank you for using the Jiskha Homework Help Forum. Many people use them interchangeably and perhaps some day that will be the norm. As far as English grammar books are concerned:

Lay means "to place something down." It is something you do to something else. It is a transitive verb.

Incorrect: Lie the book on the table.

Correct: Lay the book on the table.
(It is being done to something else.)

Lie means "to recline" or "be placed." It does not act on anything or anyone else. It is an intransitive verb.

Incorrect: Lay down on the couch.

Correct: Lie down on the couch.
(It is not being done to anything else.)

The reason lay and lie are confusing is their past tenses.

The past tense of lay is laid.

The past tense of lie is lay.