Lie, lay and lie

Test No. [color=blue]incompl/inter-28 “The Hare and the Tortoise”, question 6

The hare shot ahead and then decided to down at the side of the path for a rest.

(a) lay
(b) lie
© laid
(d) lain

Test No. [color=blue]incompl/inter-28 “The Hare and the Tortoise”, answer 6

The hare shot ahead and then decided to lie down at the side of the path for a rest.

Correct answer: (b) lie

Your answer was: [color=red]incorrect
The hare shot ahead and then decided to lay down at the side of the path for a rest.
[size=200]_________________________[/size]

Even though I know these mistakable verbs
, I usually tend to use them wrongly. Is there any clue to finally tell them apart?
Lie, lay and lie.

thanks

Lie is intransitive - no object as in The book is lying on the table. Lay is transitive - with an object as in I laid the book on the table Lie (not to tell the truth) is a regular verb lie lied lied in contrast with lie lay lain lay laid laid.

I think I’ll try to learn it by heart.