The verbs to lie (as in recline) and to lay are vexatious in their grammatical challenges. It is improper to say “I am going to lay down” or “Yesterday he laid on the hammock.”; and yet it seems that more often than not in ordinary conversation, news programs, and even political pronouncements these errors are made routinely. In the first illustration “I am going to lie down” and in the second “Yesterday he lay on the hammock.” are correct.
A verb is intransitive when it has no object: “I am”, “I smile”, or “I sleep” are all intransitive because they need no object to complete the thought. Contrast that with some transitive verbs which are followed by objects: “I threw the ball.”, “I ate lunch.”, “I built the house.” (“ball”, “lunch”, and “house” are the direct objects in those sentences).
The curve here is that the transitive verb “to lay” not only looks like its intransitive cousin, but it somewhat relates in that it involves putting an object to rest (the object in each case is in parenthesis): “I will lay the book on the table” (“book”); “He laid the brick walkway yesterday” (“brick walkway”); or “The left tackle had laid out his opponent” (“opponent”).
Comparing verbs “to lie” (intransitive) and “to lay” (transitive)
Tense To lie To lay
Present lie lay
Past lay laid
Imperfect lain laid
Participial lying laying
And, as you can see lay is the past tense of the intransitive verb and the present tense of the transitive verb. Although the rule is simple and clear, application of it has proved to be challenging.
There is no simple solution beyond getting to know both verbs well and applying them in a proper manner. My suggestion is that you avoid the past tense of to lie (“Yesterday I lay on the couch”) until you develop a comfort level. (Substitute: “Yesterday I was lying on the couch”). (Incidentally in the illustrations, “. . . on the couch” is not a direct object; it is an adverbial prepositional clause which modifies the verb lying.)
Concentrate on using the transitive verb to lay properly, which is to say only when it has a direct object; once you have mastered it, venture into the use of to lay as an intransitive verb in the past tense – – and, of course, with no direct object.
— Ken Butera