This is a practical oversimplification which will be right most of the time. For more detailed information see Mastery of the Tenses!
For new decisions.
- It's raining, I'll get my umbrella.
- I'll answer the phone!
For offers, especially new offers.
- You look tired, we'll make dinner.
- Those look heavy, I'll carry them.
For future facts.
- You'll be 42 next year.
- This time next week, I'll be on holiday.
For predictions based on opinion.
- I think David will score first.
- No one will find my hiding place.
For predictions based on 'facts'.
- The teacher will be angry, he is always angry when we are late.
- The spaghetti will be lovely, it always is!
For asking questions when the other person may not know either.
- Do you think John and Mary will get divorced?
- What time will it end? Do you know?
Used in the Consequence clause when it is probable. NOT IF CLAUSE (generally true - although it is actually possible, see Mastery of the Tenses).
In the Consequence Clause of First Conditionals.
- I will come if you invite me.
- NOT if you will invite me
- If it rains, we will not go out.
- If we work hard, we will finish it.