Does it really make sense to speak of tense with
a modal? Aren't most modals invariant? Doesn't tense
require a change of form?
We must have been there is simply modal
perfect. If must is past tense, then what do we do with must
be there? The difference between them is not tense, but aspect.
Since perfect aspect gives a sense of completion, it will seem like past
tense, but that is true in all present perfect verb phrases (like She has
eaten.)
Perhaps historically would was past
tense of will, but they seem to me very independent at this juncture
in time. I would if I could but I can't so I won't.
Surely all of these have reference to present or future time. The
difference between I would go and I will go is attitude about
the conditions under which it will happen as opposed to the certainty of
it happening. To think of one as the past tense version of the other
would be terribly misleading.
Can we think of used to as past tense
if there is no use to?
The auxiliaries in question are finite auxiliaries,
the auxiliaries which make the verb phrase a finite verb phrase (and an
arguable proposition) by grounding the statement in modality and/or tense.
By modality, I think we should understand the attitude of the speaker as
being expressed. It is generally a present tense attitude even if
toward past or future time.
She has been equals present perfect.
She might have been adds the notion
of uncertainty. It is a statement about the possibility of it having happened.
The idea would be that in the present moment of my speaking, I believe
it is possible that she has been. She should have been adds
attitudes about desirability or obligation.
We can say she had been, (past plus
perfect), and might have been (modal perfect) but there is
no equivalent for She might had been (modal plus past plus perfect)
or even might has been (modal plus present plus perfect.)
We do have periphrastic forms that
allow us to combine modality with tense and also allow us to compound
modality, but generally speaking the finite role happens only once in a
verb phrase. The finite auxiliary is also the auxiliary that moves
to the front of the clause when we turn it from statement into question.
She will go equals statement. Will she go equals question.
The mood element (grammatical subject plus finite verb) is at the heart
of what makes a sentence a sentence in traditional grammar, a much better
way of talking about it than the grossly misleading complete thought.
There may be competing ways to analyze this,
but this is the one that makes the most sense to me.