to be or not to be...

[Antonella]

Hi there fellow collegues,

I'm stuck on yet another grammatical question, but this time it involves
sentence modality and its contents.

If I were to say: "You eat cookies", the main verb in the present tense is
"eat" and this is a declarative sentence.
If I were to say: "Eat your cookies", the main verb is still "eat" and this
sentence is in the imperative mode.  However, is "eat" in the present
tense? Is the imperative, then, always in the present tense?  From my
readings, the imperative is formed from the present and therefore, the
answer to my question should be yes.  Any other opinions?

Now, here is part two.  If I were to say: "Be careful", we still have a
sentence in the imperative mode, but this time with the main verb "be".
However, what is "be" derived from?  It surely is not a "present form" as
in the above example.  Help!

Thank you in advance,
Antonella Conte
Université Laval

P.S. Why do we say "thankS" (with an -S) and "thank you" ?