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:
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
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.)
I see Jane at the lecture tonight, I'll tell
her you want to speak to her.
you want us to stay together, we'll have to show
the world that we are responsible.
is possible for the future will to occur in a subordinate
clause, if it is reporting a question. Compare the following:
you be seeing Jane at the lecture tonight?
~ I don't know / I'm not sure if I'll see her / she'll
But if I (do) see her, I'll tell her
that you want to speak to her.
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,
got to / have to / must
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.
is no difference in meaning and little difference in usage between
must on the one hand and have (got) to on
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:
must try to save some money, if we want to
buy a house next year.
I got to go to bed now? ~ Yes, you must, if you're
going to get up early tomorrow to go fishing with Uncle
you come skating with me tomorrow? ~ Sorry, I can't.
I've got to work.
you have to wear a suit to work, or can you wear casual
clothes? ~ You have to wear a suit, I'm afraid.
also that past six months in your original sentence, Bharat,
is a colloquial way of saying for longer than.