when to use can,could,shall.....

We'll need some context. I'm not an English teacher and I'm sure someone will correct me, but this is how I see it. Since I'm not an expert in this area, I'd call these guidelines but not hard and fast rules:

Will means you're going to do something. It's not only future tense but also implies a positive. I will walk tomorrow.

Would is future conditional. I would be lying if I said I was walking tomorrow. I would like to walk tomorrow.

Can refers to ability. I can walk.

Could also, in a way, refers to ability although it also implies either future tense or a decision being made or a conditional or even, sometimes, past tense. I could walk tomorrow, if my schedule was free. I could walk, if I wanted to. If I could walk, I would. I could walk if my mother took the car.

May means not just ability but that permission has been granted. But it does also imply a possible future decision. I may walk, my mother said it was all right. I may walk if I wish to.

Might is very similar to may but it doesn't seem to have as much of a permission aspect to it. I might walk, I might not. It depends.

Must implies a requirement. I must walk or I will gain weight.

thanks very much for ur reply sir

I'm a ma'am. You're welcome.