Grammer have to /must / should

must, have to and have got to: expressing the present

Must, have to and have got to are all used to express obligation or the need to do something.

They can be used interchangeably in the present tense, except that must suggests that it is the speaker who has decided that something is necessary, whereas have to and have got to suggest that somebody else has imposed the decision.

Have got to is characteristic of very informal speech. Have to sounds slightly more formal.


Compare the following:

I must clean the house before mum gets back. I want her to find it all neat and tidy.
Sorry, I can't come out now. I've got to tidy up my room before I'm allowed out.
He has to attend the clinic every two weeks. He's really quite seriously ill.
You must come and visit us again soon. It's ages since we saw you.


With frequency adverbs such as always, often, sometimes, never, etc, have to is normally preferred:

I usually have to work on Saturdays so I hardly ever go away for the weekend.
They sometimes have to get their own suppers if their mother is working late.


must and have to: expressing the future and the past

Must and have got to have no future or past tense forms.

We cannot say: I had got to.../ I'll have got to.../ I'll must.../ I've must....

However we can also use must to express future as well as present intention, especially if it is the speaker who decides that something is necessary. But it cannot be used to express past intention.

Have to is the only one of the three that possesses past and future forms.

Compare the following:

To get to Leeds by ten, I shall have to leave London before six tomorrow.
To get to Leeds by ten, I must leave London before six tomorrow.
You'll have to put the scaffolding up before you go on to the roof. It's not safe otherwise.
You'll have to have that tooth extracted. It's very badly infected.
We had to leave the party early. Tom was obviously unwell.
We've had to cancel our holiday. Tom is just not well enough for a walking holiday.

must, have to and have got to in the interrogative

Have to and have got to are often preferred in the interrogative, especially if the obligation is imposed from the outside.


Compare the following:

What time have you got to be back? ~ Dinner's at seven. So by half past six really.
How often do you have to travel to America on business?
~ About once every six months.
Must you leave right now? Won't you stay a little longer?
Do you have to leave now? ~ I do, unfortunately. I've got to collect my son from school.


have to and mustn't

We have to use have to for the negative of must when there is no obligation or necessity to do something:

You don't have to drink champagne at the reception. You can have a soft drink.
I didn't have to play after all. Jane turned up and could partner Alice.
You won't have to drive Tom to the airport next Saturday. Julie's taking him.
We use mustn't to say that something is not allowed

You mustn't drink if you're going to drive afterwards.
You mustn't drink that water. It's contaminated.
You mustn't lie under oath. If you do, that's perjury.
I mustn't forget my keys. I'll put them here so that I remember them.



It’s not quite right syntactically, Hani. To express this idea, you need should have + past participle, so it should read:
'I should never have left my job.'
For the if clause, you need the past perfect. The main clause would then read:
'I should never have left my job. If I had stayed with BP, I would have won promotion and I would be a rich man now!'



Note that if we wish to give emphasis to the condition and make it sound more dramatic and formal, we can omit if in the subordinate clause and invert subject and verb:
'Had I stayed with my previous job, I would have won promotion and we wouldn’t be in this unfortunate position now.'
Consider the following:
'I shouldn’t have gone to Jane’s place for the weekend. If I’d stayed in London, we could have worked on that report and it would be finished by now.'

'I should never have agreed to take that parcel on the plane for him. But I had no idea what was in it. If only I had said ‘no’, I wouldn’t be in prison now.'



In the examples above, note that if only is used to express a strong wish or regret and that could have suggests a probable or possible outcome (cf. might have.)

Here are some further examples of the latter:

'We might have won the match, if Beckham had been playing from the beginning.'

'I might have gone to school in America, if my parents hadn’t moved to Singapore.'

'We could have finished that cup of coffee, if you hadn’t insisted on us being here five minutes early.'

'If cholera had been diagnosed earlier, his life could have been saved.'
Note the further variations in the above sentences. In the final example, it is convenient to use the passive voice instead of active voice: 'If they had diagnosed cholera earlier, he might not have died.' And in the David Beckham example, there is a slight preference for past perfect progressive rather than past perfect to emphasise the continuity aspect.


Finally, do you remember those first three uses of the conditional?

Conditions that are generally true, where we use the present simple in both subordinate and main clause I stay inside if the temperature falls below –25°C. I just don’t go out.
Situations where we are predicting a future event if the condition is met and where we use future reference in the main clause and the present simple in the subordinate clause You’ll feel much better tomorrow if you stay at home today
Conditions where something unreal is being discussed and
where we use would in the main clause and past simple in the
subordinate clause Would you accept a diamond ring if he offered it to you? I wouldn’t!

conditionals: future reference

I think that the grammatical rule that you are referring to, Bharat, is the one that states: when we want to talk about the future in a conditional way, the verb in the subordinate if-clause remains in the present tense and the verb in the main clause is in the future, normally will + infinitive. This is a very common pattern. Compare the following:

If it's cold and wet next Saturday, I shall stay at home. I shan't be playing golf.
( NOT: If it will be cold and wet next Saturday, I shall stay at home.)
If he doesn't let me know by tomorrow, I'm going to cross his name off the list.
( NOT: If he will not let me know by tomorrow, I'm going to cross his name off the list.)
If I see Jane at the lecture tonight, I'll tell her you want to speak to her.
If you want us to stay together, we'll have to show the world that we are responsible.
It is possible for the future will to occur in a subordinate clause, if it is reporting a question. Compare the following:

Will you be seeing Jane at the lecture tonight?
~ I don't know / I'm not sure if I'll see her / she'll be there.
But if I (do) see her, I'll tell her that you want to speak to her.
We can use do in the above sentence for contrastive emphasis, i.e.: It's not very likely that I'll see her, but if I do see her,…









conditionals: general condition

Note that if we are stating a general truth, the verb in both clauses normally remains in the present. This was probably Amy's intention, when she was talking about the things people need to do if they want to stay together. Compare the following:

If I drink coffee in the evening I cannot sleep at night.
You can walk on the grass if you want to.
We prefer to sleep outside at night, if the temperature stays above 30 degrees.
If you want us to stay together, we must demonstrate that we can keep this place tidy.
If you want to learn a musical instrument, you must be prepared to practice for an hour each day.
Note that when stating a general truth, we can sometimes replace if with when or whenever:

When I drink coffee in the evening I cannot sleep at night.
We prefer to sleep outside at night, whenever the temperature stays above 30 degrees.
When you are learning to play a musical instrument, you must practice for half an hour each day.



have got to / have to / must

Note that we've got to in your examples, Bharat, is not a reference to the present perfect. We've got to here is used as an alternative to we have to or we must to express obligation.

There is no difference in meaning and little difference in usage between must on the one hand and have (got) to on the other.

To some extent, must is used to talk about the emotions or wishes of the speaker or hearer, whilst have (got) to is used to discuss obligations that are imposed from outside by some external body. Compare the following:

We must try to save some money, if we want to buy a house next year.
Have I got to go to bed now? ~ Yes, you must, if you're going to get up early tomorrow to go fishing with Uncle Bill.
Can you come skating with me tomorrow? ~ Sorry, I can't. I've got to work.
Do you have to wear a suit to work, or can you wear casual clothes? ~ You have to wear a suit, I'm afraid.
Note also that past six months in your original sentence, Bharat, is a colloquial way of saying for longer than.