What about this sentence puts it in the past tense?
The -t of beszélt.
In general, the sign of the past tense is -t-.
Could this also be [formal] you did not speak enough, or must is start - O nem... [+ accent on O]
It can be both. And the way to differentiate is context. When you are formally talking to somebody, who just gave a speech, it is obviously the formal you. And if you are talking about a third person, then that's that.
is elég a noun in this context? and that's why it has an accusative ending?
could you also say nem elég beszélt or is that different/wrong? Thanks!
Eleget is more like an adverb by its meaning. The accusative can have adverbial uses, think of "kicsit".
Yes, you have to use the accusative ending -et. The form 'Nem elég beszélt' is wrong.
Correctly: nem eleget beszélt
The contraction -- didn't -- was not accepted. Should it be?
(He didn't speak enough.)
Of course it should.