We use will (+ base verb) when we decide to do something at the time of speaking.

eg. I forgot my bag in the classroom. I’ll go and take it.

The negative form is will not (won’t).

 

Will can also be used in these situations:

• offering to do something: You live far away from school. I’ll give you a lift.
• agreeing to do something: Can you finish the report for me? OK, I’ll do it.
• promising to do something: Thank you for sharing your secret with me. I promise I won’t tell anyone.
• asking somebody to do something: Will you please close the door? It’s a bit cold.

Shall is mostly used in the questions Shall I…? / Shall we…?

We use shall to ask for somebody’s opinion: Shall I buy that dress?

Shall is only used with I and We (first person singular and plural)