"could" being the past tense of "can".

<<The sentence: *"She could be there yesterday" is not possible for me.>>

Rick, here is a clearer example:

"I couldn't hear you yesterday", where 'could' is used as a past tense

OR, better yet:

"She couldN'T be there yesterday"

it is possible

-This is an example of where German is much simpler and without the ambiguity of English -

But German has its own oddities, like mag vs moechte.
Moechte is conjunctive of magen,but it developed an infinitive: moechten.
Also, the Konjunktiv II forms of the weak verbs are identical to the preterite forms.

I could (it has both the conditional meaning, If I could, and the past meaning...I could see it.)
I spielte (it has both the conditional meaning, wenn ich spielte and the past meaning, ich spielte)

<<I spielte (it has both the conditional meaning, wenn ich spielte and the past meaning, ich spielte) >>

This is only because, like English, the "ich" forms have merged and show no true differentiation

<<This is only because, like English, the "ich" forms have merged and show no true differentiation >>

I might add: *sometimes--they *sometimes do not show differentiation

>>But German has its own oddities, like mag vs moechte.
Moechte is conjunctive of magen,but it developed an infinitive: moechten<<

As far as I'm aware 'moechten' is not an infinitive, the infinitive form is 'moegen'.

See here:

moechten is used as an infinitive, in many grammar manuals:
http://forum.wordreference.com/showthread.php?t=25757

Yes I had a look at that before, but I don't see anywhere in any of the explanations 'moechten' being used as an infinitive. For it to be an infinitive it needs to be able to be used after a finite verb. Can it be?

'Es ist gut, Freunde haben zu moechten'.

Hmmm???