Question 1
A little girl comes up to her mother and asks if she _____ go to the park with her friend.
The girl is asking for permission, so we use may: She asks if she may go to the park. Not must, because must means it is necessary, not that someone allows it.
Question 2
If you are ill and _____ go to school, you _____ learn everything you have missed.
When you are ill, you are not able to go to school, so the first gap needs can't. Afterwards it is necessary to catch up, so the second gap needs must: you must learn everything you have missed.
Question 3
The dog had run away and the children _____ find it though they were looking for it the whole day.
The story is in the past (had run away), and they looked all day without success, so we need past inability: could not. Not can't, because that is present tense.
Question 4
As Mr. John hadn’t got the dictionary at hand, he _____ guess the meaning of the word.
He had no dictionary, so it was necessary for him to guess. For past necessity we use had to. Must and has to are present forms, so they do not fit the past context (hadn't got).
Question 5
Mark Twain _____ easily _____ across the Mississippi River.
This is about ability in the past, so we use could + base verb: Mark Twain could easily swim across the Mississippi River. Not may, which talks about permission or possibility.
Question 6
It was late but the pupils _____ stay at school for an additional lesson.
It was late but shows the pupils stayed against their wish, because it was necessary. For past obligation we use had to. Not needed, because it needs to: needed to stay.
Question 7
The children have done their homework. I think they _____ have a rest now.
The homework is finished, so the children are allowed to rest. We use may for permission: they may have a rest now. Not are able, because it needs to: are able to have.
Question 8
- Shall I retell the text?
- _____. You can only translate it.
You can only translate it shows that retelling is not necessary. To say something is not necessary, we use needn't: No, you needn't. Not Yes, you may, because the speaker is saying no.
Question 9
Patient: Must I go to the hospital?
Doctor: No, you _____. You _____ stay at home.
The answer to Must I…? when it is not necessary is No, you needn't. Then the doctor gives permission: You may stay at home. Not mustn't, which means it is forbidden.
Question 10
It _____ rain today. There are so many clouds in the sky.
The clouds show it is possible that it will rain, so we use may: It may rain today. Not can, which is not used for a possibility on one particular occasion like this.
Question 11
I understood that he _____ never _____ back.
After a past reporting verb (I understood), will usually changes to would, followed by the base verb: he would never come back. Not wouldn't, because never is already negative.
Question 12
My son is ill, so I _____ stay at home.
It is necessary because of a situation (my son is ill), so we use have to. With I we say have to, not has to.
Question 13
A: Must I do it?
B: No, you _____. It isn’t necessary.
It isn't necessary tells us the answer: No, you needn't. Not may not or can't, which mean it is not allowed.
Question 14
I feel ill and very tired, so I _____ go to school.
Because I am ill, I cannot go, so we need inability: am not able to. Not haven't to, which is not correct English (we say don't have to).
Question 15
He was very poor and _____ marry a woman eight years older than himself.
His poverty forced him, so it was a past necessity: had to. Must is used for the present and does not fit the past context (He was).
Question 16
By law, children _____ go to school at the age of 7.
By law shows it is a rule, so it is necessary: children must go to school. Not has to, because children is plural (have to).
Question 17
- _____ I trouble you for a moment?
- Yes, certainly. What _____ I do for you?
May I…? is a polite way to ask for permission, and What can I do for you? is the usual offer of help. Not What may I do for you?, which is not natural.
Question 18
He _____ know her address. Ask him.
Must is used when we are almost sure something is true: He must know her address. Not can't, which means we are sure it is not true, so Ask him would make no sense.
Question 19
You knew he was ill. You _____ have visited him.
Might have + past participle can express reproach about something someone failed to do: You might have visited him! (= you should have). Not may have visited, which only means it is possible that you visited him.
Question 20
May I take this pen? No, you _____.
A negative answer to May I…? that refuses permission is No, you mustn't (or No, you may not). Not may, because the answer starts with No.
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