We use may or could to ask for or give permission in a more formal context or situation.
May I come in? / Could you pass me that notebook, please?
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PROHIBITION:
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We use can't when something is against rurles or norms.
You can't park here, sir.
We use mustn't when the obligation comes from the person who is speaking.
You mustn't speak when the teacher is speaking.
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OBLIGATION:
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Need to, must, and have to can be considered synonyms and mean that something is required or necessary. Although there are some slight differences among them:
Need to is the one that is more limited in meaning:
1. It is used when an action should be done.
I need to clean my dirty bedroom.
2. When it is very important for someone to do something.
She didn't want to go to the city, but she needed to buy some food.
Must and have to have broader meanings, and once again the difference between them is very small.
1. We use have to when the obligation comes from somebody else. The speaker can't change that because it's a law or a rule.
We have to wear an uniform at our school.
She can't come to the party, because she has to travel tomorrow early in the morning.
2. We use must to show that the obligation comes from the speaker. It isn't a rule or a law.
I must call my grandfather tonight.
You must come and visit us the next time you come to Seville.
HOWEVER, BOTH (MUST & HAVE TO) CAN BE USED ALMOST INTERCHANGEABLY IN THE FOLLOWING CASES:
1. When something is required or neccesary. We must / have to go to the post office.
2. When something is required by law. You must / have to pay your taxes.
3. When something is very probable. It must / has to be 45 degrees here.
IN ALL THESE CASES, MUST IS MORE FORMAL THAN HAVE TO.
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NO OBLIGATION:
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We use don't have to when there is no obligation to do something. It means that you can do something if you want, but it is not obligatory.
We don't have to wear an uniform at our school. We can wear it if we want to but it's ok if we don't.
Here there are some websites where you can practice these confusing modals:
Conditional Clauses are used to express that the action in the main clause (without if) can only take place if a certain condition (in the clause with if) is fulfilled. There are four types of Conditional Clauses.
THE ZERO CONDITIONAL
FORM:
USES:
1. For things that are always true.
Ex.: If you heat water, it boils.
2. For habits or things that usually happen, we can also use the present continuous in the if clause. In this case you can replace 'if' with 'when' and the meaning is the same.
Ex.: When I'm feeling sick, I go to the doctor.
3. To give instructions or invitations which depend on certain conditions we use the present simple + imperative.
Listen to this song by Bruno Mars and find the different First Conditional Clauses.
USES: This type of conditional is used to talk about future possibilities. 'Will' is a modal verb, but you can use other modal verbs, for example 'might' and 'could' in the result clause.
Ex.: If you ever find yourself stuck in the middle of the sea, I'll sail the world to find you.
Sometimes you can use 'if' to introduce a condition and 'whether' in the following cases:
1. To present two alternatives.
Ex.: Tell me whether you're coming to the concert or not.
2. After prepositions.
Ex.: Let's talk about whether Bruno Mars is a good singer.
3. Before infinitives.
Ex.: I don't know whether to go to the cinema.
4. When it starts the subject clause.
Ex.: Whether you love or hate him is none of my business.
When we use the first conditional clauses, we can also use 'unless'. This means 'except if' and is the negative of 'if'.
Ex.: Unless you study hard, you won't pass the exams.
