We use I’ll … (= I will) when we’ve just decided to do something. When we say ‘I’ll do something’,
we announce our decision:

• Oh, I left the door open. I’ll go and shut it.
• ‘What would you like to drink?’ ‘I’ll have orange juice, please.’
• ‘Did you call Max?’ ‘Oh no, I forgot. I’ll call him now.’

• I’ll phone him now. (not I phone him now)

• I’m a little hungry. I think I’ll have something to eat.
• I don’t think I’ll go out tonight. I’m too tired.

• I can see you’re busy, so I won’t stay long. (= I will not stay long)

We oft en use I’ll in these situations:

• That bag looks heavy. I’ll help you with it. (not I help)

• a: Can you give Tom this book?
  b: Sure, I’ll give it to him when I see him this aft ernoon.

• Thanks for lending me the money. I’ll pay you back on Friday.
• I won’t tell anyone what happened. I promise.

• I’ve tried to give her advice, but she won’t listen.
• The car won’t start. (= the car ‘refuses’ to start)

• Will you please turn the music down? It’s too loud.

We do not use will to talk about what has been decided or arranged before:

• I’m going on holiday next Saturday. (not I’ll go)

• I’m meeting Kate tomorrow morning. (decided before)
• a: I’ll meet you at half past ten, OK?
  b: Fine. See you then. (decided now)

We use shall mostly in the questions shall I … ? / shall we … ?

• Shall I open the window? (= do you want me to open it?)
• I’ve got no money. What shall I do? (= what do you suggest?)
• ‘Shall we go?’ ‘Just a minute. I’m not ready yet.’
• ‘Where shall we have lunch?’ ‘Let’s go to Marino’s.’

Compare shall I … ? and will you … ? :

• Shall I shut the door? (= do you want me to shut it?)
• Will you shut the door? (= I want you to shut it)