strong and weak forms

I'm a Polish student and I have huge difficulties with that subject. I would like to find out in what context we use strong or weak forms, are there any instuctions which would help me?

I think you mean verbs like burn, burnt, burnt or burn, burned, burned? You can usually choose the regular or irregular form as you please. Americans often prefer the regular forms of these verbs. There are cases where only the regular form is correct: a learned man. Here learned  is used as an adjective and learnt is wrong.

The regular forms of some verbs are not very often used; speed, speeded, speeded is one of them.

CB