Hebrew tends to use what is known as "plural of majesty" to indicate
supremacy. Even though the forms of Elohim and Adonay were plural, they
would frequently be used with singular verbs to distinguish them from
actual plurality of meaning. For instance, in Gen 1:1, we might
conclude that in the beginning "gods" created the heavens and earth, but
"created" is in a singular form. Since there is no other way to take
Elohim than as the subject of the sentence, we begin to see how it was
used as a singular term. Thus, the simple form of the noun itself is
not enough to tell us whether God or gods is the expected
translation--we must use the form of the verb and other indicators in
the context to decide. You could probably get a better explanation from
b-hebrew.
As for Theos, this was the typical Greek word for God. It was the word
used in the LXX to translate Elohim, and it was used as we would
expect--plural when gods were plural and singular when God was
singular. The real key to this issue is why Hebrew sometimes uses
plural for a singular individual, which is why you might want to bring
it up on b-hebrew. One more point, though: It has been suggested that
the plurality of Elohim indicated or allowed for the Trinity concept in
a latent form. While I would say that it does allow for it, there is no
necessary implication that the OT writers understood this. Also, if we
give too much weight to this suggestion, then the NT's failure to use a
similar plural term would seem to be a contradiction.
Trevor