Could, of course, functions as the past tense of can, and like other
past forms, it sometimes simply indicates past time: In those days
there was no security and anybody could walk in, day or night. But,
like the past forms of other verbs, it can also indicate things like
tentativeness, indirectness, deference and a wish not to impose.
Compare
12
The basic difference between shall and should is that “should” is the
past tense of “shall.” but when we use these words or models, the
usage is not as simple as using “should” in place of “shall” in the past
tense. “Should” is not used in the past tense independently; it has either
present or future reference when it predicates a main clause
. “Should” is the conditional form used for “shall.” Occasionally it is
used as a past tense of “shall
13
it almost the same but i think that must use when nessary the most
14
would is the past of will we use will when we talk about the future