There are two main issues with the verbs lie and lay (infinitive forms are to lie and to lay).
Now, I think most people understand that when you are on your back, more or less in a horizontal position, you are lying on something (either that, or you’re floating on air).
I am lying on the couch.
Tomorrow I will lie on the floor.
I am lying in my chicken’s nest box. I cannot say I am laying in my chicken’s nest box unless I am, perhaps, laying an egg, or maybe I am laying a bowl of corn grit for the chicken to eat. (In which case, I would say that I am laying a bowl, etc., in the chicken’s nest box.
If you are laying, you are laying something (down). You are laying the guacamole on the counter. You are laying the plates on the table. You are a chicken and are laying eggs in your nest. Actually, the chickens can just be laying. We don’t have to say where the eggs end up …. the chickens just lay. That’s cool, but it’s a different word than the lay that people do.
You can also lay yourself down, e.g., “Now I lay me down to sleep.” When you do that, however, once you have completed the process of laying yourself down, you are then lying down. In less than a breath, you can go (in fact, you must go ) from laying to lying. How exhilarating! Is this a great language or what?
The second thing that many native writers/speakers don’t realize is that the past tense form of lie is lay.
WHAT? How can this be! Why would someone design a language with such potholes?
RIght now I lie on the carpeted floor. Yesterday, I lay on the floor. And (get this), right now I am lying on the floor; yesterday I was lying on the floor, too. Yes, indeed. In fact, I lay on the floor all day yesterday! But today, my plan is to lie on the floor. In fact, I will lie on the floor so well today that, by the end of the day — considering that the past participle of lie is (new information here, ha ha ha) lain, I will be able to say I have lain on the floor the entire day.
And just when you think you have everything figured out, your dog, standing on his hind quarters, comes up to you at 7pm, begging for dinner. “But I fed you at 5pm,” you proclaim. Still standing, the dog shakes his head. Then you realize your dog isn’t lying. In fact, he’s telling the truth. No lie.