Ability
We have enough milk in the fridge so we (must, mustn't, needn't) buy some more.
We have enough milk in the fridge so we needn't buy some more.
_____ I call you later?
Can/May I call you later?
If you leave the stove on, the house (may, might) catch on fire.
If you leave the stove on, the house might catch on fire.
If you get in trouble, I (won't wouldn't) help you.
If you get in trouble, I won't help you.
He will be having lessons for another two hours. We (must, mustn't, needn't) visit him now.
He will be having lessons for another two hours. We mustn't visit him now.