Modal Verbs Summary

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5–) It’s possible to use “BE ABLE TO” instead of “can” to express ABILITY in every situation.

E.g. I am able to play the piano = I can play the piano.

6–) Use “BE ABLE TO” as the INFINITIVE FORM of “can” when it is necessary, like in a perfect tense.

E.g. I haven’t been able to sleep recently. = It hasn’t been possible to sleep recently

(“I haven’t can sleep recently” – It’s wrong – never use a modal verb after another one).

7–) Use “WAS ABLE TO (DO)” to talk about a SPECIFIC SITUATION that someone MANAGED. E.g. The building was on fire but my friend was able to escape safely. = He managed the situation to escape safely. (It wasn’t an ability of escaping, just a particular situation).

8–) Use “COULDN’T” as the NEGATIVE PAST of CAN in every situation.

E.g. My grandfather couldn’t speak Portuguese very well. = He didn't have this ability.

Unfortunately he couldn’t escape from the building on fire. = He wasn’t able to manage that situation).

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2–) Could

Aff: I Could (do). Possible Future Happening: There Could (be).Neg: I Couldn’t (do). Past of Can (General Ability): I Could (do).Quest: Could I (do)? Past Simple of Can (was possible): I could (do).

Past (Ability/Opportunity/Possibility that didn’t happen): I Could have (done). Past Neg (Something didn’t happen and wouldn’t be possible to happen): I Couldn’t have (done).

1–) Use “COULD” as the past of “CAN” to talk about the ABILITY that someone had.

E.g. In 1910 the best runners of the world could run 100m in 10.8 seconds. = They had the ability of running 100m in this time.

2–) Use “COULD” to ASK SOMETHING in a politer way than with “CAN”.

E.g. Could you take a glass of water to me please?

3–) Use “COULD” in the present to MAKE A SUGGESTION. It’s possible to use “CAN” instead of “could” to make a suggestion, but can is more sure it’s almost an invitation.

E.g. A: What shall we do tonight? B: We could watch a movie. (= it would be nice to watch a movie).

A: What shall we do tonight? B: We can watch a movie. (= let’s watch a movie?).

4–) Use “COULD” to talk about a HYPOTHETICAL SITUATION (sometimes in a hypothetical future, sometimes in the present), in this case could means: WOULD BE ABLE TO.

E.g. Why don’t you enter in the contest? You could win it. = **You would be able to win the contest. He works about 16 hours a day. I couldn’t be able to do so. = I wouldn’t be able to do so.

5–) Use “THERE COULD (BE)” to talk about POSSIBLE FUTURE HAPPENINGS. It’s possible to use “THERE MAY BE” or “THERE MIGHT BE” instead of “there could be” to talk about it, but “could” is more unsure than “may” and “might”.

E.g. The weather is unstable there could be a rain tomorrow. =There is a possibility of raining tomorrow.

The weather is unstable there may/might be a rain tomorrow. = The possibility is higher.

6–) Use “COULD HAVE (DONE)” to say that someone/something HAD THE ABILITY / OPPORTUNITY / POSSIBILITY to do something but DIDN’T do or happen.

E.g. Why did you stay in hotel? You could have stayed at our friend’s house, he has a free bedroom there. = You had the opportunity of staying in our friend’s house but you didn’t.

7–) Use “COULDN’T HAVE (DONE)” to say that something DIDN’T HAPPEN AND even though you wanted or tried to do so IT WOULDN’T BE POSSIBLE.

E.g. When I’ve been to the city where our friend lives I decided not to stay at his house, but then I discovered he wasn’t there so I COULDN’T HAVE STAYED at his home. = Even though I had wanted to stay there it wouldn’t be possible.

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3–) Must

Aff: I must (do). Past: I Must Have (Done).Neg: I mustn’t (do). Present Continuous: I Must Have Been Doing.Quest: Must I (do)?

1–) Use “MUST” to say that YOU ARE SURE about something.

E.g. You’ve been working all day, you must be tired. = I’m sure you are tired

2–) Use “MUST” to give a STRONG ADVICE about something or when it’s really NECESSARY TO DO something.

E.g. Wow, it’s freezing, you must wear a coat otherwise you’ll catch a cold. = Strong advertisement.

You are at the library, you must be quiet = It’s necessary to be quiet.

3–) Use “MUSTN’T” to say that it is necessary that you DON’T DO SOMETHING.

E.g. You mustn’t steal thing in a store, you can be arrested.

4–) Use “MUST HAVE (DONE)” when you are SURE THAT SOMETHING HAPPENED use MUST HAVE BEEN DOING as the past continuous form.

E.g. The phone rang but I didn’t hear it, I was too tired so I must have fallen asleep. = I’m sure it happened.

My roommate told me my phone was ringing but I didn’t hear it, I’d just got home, so I must have been taking a shower then. = I’m sure it was happening then.

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4–) May / Might

Aff: I May / Might (do). Possible Future Happening: There May / Might(be).Neg: I May not / Mightn’t (do). Was Possible in the Past: I May / Might have (done).Quest: May / Might I (do)? Past Continuous: I May/Might Have Been Doing.

1–) There is no important difference