The auxiliary verbs that can be used to express complete certainty are shall, will, can, could and must.

• There is the phone. That will be Ram.
• I shall come tomorrow. OR I will come tomorrow. (Both shall and will can be used with I and we. There is no difference in meaning.)
• He will come.
• It will rain this evening.
• It won’t rain this evening.
• There is the doorbell. Who can it be? Well it can’t be James – He is in Delhi.
• You must be tired. I will get you something to drink.
• I will buy a car if I get my rise.
• She will come if you invite her.

Expressing probability and possibility

We can use the auxiliary verbs should, ought, may and might to talk about possibility and probability.

• She should be here soon.
• She may come.
• It may rain this evening.
• We may go camping next week.
• She ought to know better.
• I may buy a car.
• You should be able to solve the problem.
• She may buy that house.
• I should be able to finish this job by Monday.

Notes

Might shows less possibility than may.

• She might come. (Perhaps a 20% chance)
• She may come. (Perhaps a 50% chance)