• Diane is working next week. (not Diane will work)
• Are you going to watch anything on TV this evening? (not Will you watch)
  For 'is working' and 'Are you going to ... ?', see Units 19-20.

But often, when we talk about the future, we are not talking about what somebody has decided to do. For example:

She'll pass does not mean 'she has decided to pass'. Joe is saying what he knows or believes will happen.

He is predicting the future.

When we predict a future happening or situation, we use will/won't.

• They've been away a long time. When they return, they'll find a lot of changes here.
• ‘Where will you be this time next year?’ ‘I'll be in Japan.’
• That plate is hot. If you touch it, you'll burn yourself.
• Tom won't pass the exam. He hasn't studied hard enough.
• Anna looks completely different now. You won't recognise her.
• When will you get your exam results?

We often use will ('ll) with:

• I hope Kate passes the exam. (or I hope Kate will pass ... )
• I hope it doesn't rain tomorrow.

I shall ... / we shall ...

• I shall be late this evening. (or I will be)
• We shall probably go to France in June. (or We will probably go)

• We'll probably go to France.

• I shan't be here tomorrow. (or I won't be)

• She will be very angry. (not She shall be)