Will is a complicated word. Calling will VERB “the future tense” is beginner's grammar which will not stand up in many contexts.
- (In case you’re worried, I’m not going to open the contentious question of whether it’s permissible to call a periphrastic construction a ‘tense’.)
To begin with, it is not by a long shot the future tense; it is at most a future tense, alongside shall VERB, be going to VERB, and the simple present of VERB.
Next, will VERB does not necessarily mark a clause as having future reference. In many if .. then constructions (although not this one), it implies consequence, not subsequence, logical rather than temporal ‘following’:
If you add one to itself you will get three.
If he’s in Dallas he will be at the Ritz-Carleton.
In other cases will is employed in clauses which do have future reference, but the futurity is inherent in the proposition itself, not in the verbal construction. The if you will sit here is an instance: futurity is implied by for ten minutes, not by will, which is why the simple present is just as acceptable here.
In this particular case, will is used, in both if you will sit and I will tell, in its older sense of be willing.
If you are willing to wait here for ten minutes, I will* (consequential, not futurive!) be willing to tell the manager …
The same sense, with somewhat enhanced courtesy, may be expressed with a conditional past:
If you would sit here for ten minutes, I will tell the manager …
The use of "had been" sounds a bit unnatural in the sentence. I think you should use either used to be or was as follows:
I used to be/was an introverted person until I went into/joined the army.
Best Answer
Connective prepositions before, after, when, while, as soon as, etc. may be added to a clause to express that a second activity occurs earlier than, later than or at the same time as the activity in the main clause.
If the verb in the main clause is present tense, the verb in clause following the preposition is also present.
Connective prepositions may also be added to a clause with a future tense verb expression such as will or (be) going to. In this case, the verb in the clause following the connective preposition does not change tense. It remains in present tense form.
So, the correct choice will be
The difference between variants A, B, and C is that only variant A is grammatically correct.