Lay and lie are two verbs having a similar meaning but used in different contexts. They all mean rest in a horizontal position. Now let’s consider the right use of each.
Lay( present participle: laying, past and past participle: laid) is a transitive word, that is, it takes a direct object*.
Examples: 1. The men laid the dead man in the coffin.( This sentence is in the past tense ;hence the past tense of lay, which is laid, is used)
2. He laid the book on the table.
3. Desmond, lay the bed on the floor.
Lie( present participle: lying, past participle: lain, past: lay) is an intransitive verb, that is, it does not take a direct object.
Examples:1. The dead man lay on the floor. ( Take note “lay” as used in this sentence is the past tense of lie, and “the floor” in the sentence is not a direct object. It is just an adverbial phrase describing where the man lay. )
2. Please, lie down on the sofa.( not lay)
*A Direct object directly receives the action of the verb in the sentence. For example: “me” in John beat me is a direct object because it receives the action directly.
ARE YOU CONFUSED?
JUST TAKE NOTE: YOU LIE DOWN BY YOURSELF BUT YOU LAY SOMETHING DOWN. As a result, it will be wrong to say: I am laying down but instead you say: I am lying down. However, you can say: I am laying it down, not I am lying it down.
Grammar Rules 