Making Guesses is a Good Habit -

Modal Verbs of Probability for New English Learners..

Listed below are examples and uses of modal verbs of probability. Modal verbs of probability are used to express an opinion of the speaker based on information that the speaker has. Example: He must be at work, it's 10 o'clock. In this case, the speaker is 100 % sure that the person is at work based on the speaker's knowledge that the person in question usually works at during the day.

Must 

Use 'must' plus the verb when you are 100% (or almost 100%) sure that something is the case. 

Present = must + verb (do)

They must be in Bangalore by now. They told me they were going last week.
Josh must think I'm crazy because I think grammar is easy!

Past = must have + past participle (done)

Tara is smiling. She must have done well on the test.
Alisha must have asked for some help on the test because she got an A.

Might / May

Use 'might' or 'may' to express an opinion that you think has a good possibility of being true.

Present = might / may + verb (do)

She might come this evening, but she also had some work to do.
Devendra may invite Jutika to the match. I know he really likes her.

Past = might / may + have + past participle (done)
Jayshree might have gone to Goa for her vacation. I think he wanted to practice Portugese this summer. 

Could

Use 'could' to express a possibility which is one of many. This form is not as strong as 'might' or 'may'. It is just one of a number of possibilities.
Present = could + verb (do)
Jannat could be at work, or she could be at home. I'm not sure.
We could hire that company or the other. It doesn't really matter. 
Past = could have + past participle (done)
Pratim could have arrived late. I know he missed the bus.
Alisha was tired. She could have stayed at home today, or she might have gone to work. 

Can't / Couldn't 

Use 'can't' to express an opinion that you are 100% sure is NOT true. We use 'must be' or 'must have been' if we're sure in a positive sense, but 'can't be' or 'can't have been / couldn't have been' if we are sure in a negative sense. Note that the past form remains 'can't have done' in British English, but changes to 'couldn't have done' in American English.
Present = Can't + verb (do)
You can't be serious! I'm not going to loan you Rupees /-1 million!
Pratim can't like that show. He doesn't enjoy comedy. 
Past = Can't / Couldn't + have + past participle (done)
They can't have worked until late because they were on time for the meeting.
She couldn't have believed that story. She knows he's a liar!

Modal Verbs of Probability Quiz

Use must, might, may, could or can't plus the correct form of the verb. In some cases, there is more than one correct answer. Pay close to attention to time words in order to conjugate the modal verb of probability correctly.

1. Where is Vishal? He __________ (be) at school. Classes begin at 8 and he's never late.
2. She __________ (think) that it is a good idea. It's crazy!
3. I'm absolutely sure! They __________ (arrive) yesterday. Tonmoy showed me his train ticket.
4. Courses __________ (begin) the fifth of September, but I'm not really sure.
5. Are you joking! Arjun __________ (go) to Delhi last week. He doesn't have enough money to go to Nepal. 
6. They __________ (live) in New Delhi, or he __________ (be) in Mumbai. I know he likes big cities.
7. The concert __________ (be) wonderful last night. Josh is a fantastic singer. You __________ (have) fun.
8. Students __________ (get) sick and tired of grammar. I know it's kind of boring. 
9. Lata __________ (be) looking for a job because she finished college last year.
10. Debjani __________ (want) to get in touch with you. She's always asking about you when we talk. 

Answers

1. must be - Vishal is never late.
2. can't think - I think it's a crazy idea, so I'm sure.
3. must have arrived - I'm absolutely sure they arrived yesterday because I saw the ticket.
4. could begin - It's possible, but I really don't know.
5. can't have gone / couldn't have gone - Arjun doesn't have money so it isn't possible in my opinion.
6. could live / might live / could be / might be - I know he likes big cities, but I don't know for sure.
7. must have been / must have had - Josh's a fantastic singer, so I'm sure you had a great time.
8. must get - I'm a teacher. I know!
9. must be / might be - It's logical that she's looking for a job.
10. must want - I know that she thinks about you often. 

Happy Studying !!!