I haven’t thought about the difference between lie, lay, laid, lain since high school English class, but in writing a bit of story the other day I used ‘lay’ [correctly it turns out], but it didn’t sound right. So to be sure, I did a search and found a brilliant, brief explanation from Encyclopaedia Brittanica. I’ll paraphrase but all credit to them …

First of all, when I say ‘lie’ I mean lie as in ‘lie down’ not tell a falsehood. 

Simply put, use ‘lie’ when it is an action with no object. It’s something you do yourself, in other words. For example:

“I always lie down after lunch for a nap.”

“She lies down to reach beneath the sofa.”

“They lie down together to mediate.”

On the other hand, use ‘lay’ when you take action in regards to an object. For example:

“Put down the book and lay it on the table before answering the door.”

All the above examples are in the present tense, but some confusion arises when we consider the past tense. Why? Because ‘lay’ is the past tense of ‘lie’! For example:

“After lunch, I lay down for a nap.”

But ‘laid’ is the past tense of ‘lay’, so …

“She laid the book on the table before answering the door.”

So what about ‘lain’? That is the past participle tense of lie. You would use it this way:

“I had lain on the sofa much longer than I intended.”

The past participle of lay is still ‘laid’ so it would be used this way:

“She had laid the book on the table before answering the door.”

And just to finish things off, the present participle tense of ‘lie’ is ‘lying’ and for ‘lay’ it is ‘laying’. They would each be used this way:

“I am lying down for a nap after lunch.”

“She is laying the book on the table before she answers the door.”

I hope this was helpful! Happy writing and productive editing! I’m going to lie down now….