Lay vs. lie

The following is an excerpt from the book
The Careful Writer: A Modern Guide to English Usage
by Theodore M. Bernstein.

Lay is a transitive verb and never anything else except in the meaning of to deposit eggs ("The hen is laying") and in a few nautical terms (lay aloft.) For those whose grammar lessons are far behind them or forever incomprehensible, let it be explained that a transitive verb is one that acts as a transmission belt, conveying action or influence from a subject to an object. The present tense is lay ("Watch as I now lay this book on the table"). The past tense is laid ("He laid his watch carefully on the desk"). The present participle is laying ("He is laying his plans for a medical career"). The past participle is laid ("He had laid his plans before entering college").

Lie is an intransitive verb and never anything else. An intransitive verb is one that confines the action to the subject; it does not transmit the action or influence to an object. The present tense is lie ("The Azores lie in mid-Atlantic"). The past tense is lay ("He lay in pain waiting for help"). The present participle is lying ("He is lying in bed"). The past participle is lain ("He had lain there for several hours").

The most common errors are, first, the use of the forms of the transitive lay where forms of lie are required-- "The pencil was laying on the table" (never, unless the pencil were depositing eggs) or "Women laid down in the roadway to halt the Soviet tanks" (No, down isn't laid; it comes off a duck) --and second, the use of laid instead of lain for the participle form of lie-- "He had laid there for several hours."

These confusions are not infrequent, but the errors can only be classed as illiterate.