Difference between lay and lie.

Present           Past            Past participle
  Lay                Laid            Lain
  Lie                 Lay             Lain

How to remember the difference?

Lie -- Lie down on something.

Lay -- Lay something on the table.

Eg.
You lay something down.
People lie down.

I think pupils should remember the past tense form to avoid confusion. So, just concentrate on the laid and lay.

She lay down on the sofa.  (Past tense)
She laid the cloth on the table. (Past tense)

For more information, refer to grammar girl website.
/grammar.quickanddirtytips.com/

Mr Pang