Used to is an idiom that has a very special presupposition and some unusual properties.
First, it's always pronounced /yustə/ or /yustu/, never /yuzd tu/:
- I used to be afraid of snakes. /ay 'yustəbiyə fredə 'sneks/ [NOT /'yuzd/]
Second, English has two idioms pronounced this way -- one, which always appears as a predicate adjective with some form of be -- means 'accustomed to'. That's not the one under discussion here.
- I am used to driving in the left lane.
Third, the particular used to idiom under discussion here can appear in any sentence, with the main verb (in infinitive form) following it:
- He came/comes here often. ~ He used to come here often.
- Carter is/was President. ~ Carter used to be President.
(Since infinitives have no tense, they can refer to either a present or past tense verb.)
Fourth, these used to constructions refer to both present and past tense sentences simultaneously. However, they don't refer in the same way; the past is asserted (i.e, claimed by the speaker to be true), while the present is presupposed (i.e, assumed by the speaker -- and normally the listener as well -- to be true).
That is, if one says that Carter used to be President, then one is saying that
- the past sentence Carter was President is True
and also presupposing that
- the present sentence Carter is President is False.
And if you did say that, and then someone said "You're wrong!", what would they be denying? One can only deny an assertion, so they'd have to mean that Carter was never President. They couldn't possibly mean that Carter is still President.
This can work both ways -- if you said, instead,
- Carter isn't President any more.
and someone said you were wrong, they'd have to be denying that he was not President now.
One more reason why there are usually several different ways to say things.