The OED provides quite a mouthful on though as an adverb. We can imagine the lecturer tapping his pointer on the chalkboard as he intones that though is:
An adversative particle expressing that relation of two opposed facts or circumstances (actual or hypothetical) in which the one is inadequate to prevent the other, and therefore both concur, contrary to what might be expected.
Most important is that when though is used as an adverb, it modifies a verb. Whenever though is used to modify a verb and to talk about the opposite of what came before, or something different than what was expected, it's an adverb. In English, this often comes at the end of a sentence.
The OED also provides a list of meanings (or of words and phrases that can sometimes be substituted for though with the same sense):
- For all that
- in spite of that
If a sentence is composed like this, though is an adverb:
I expected him to lose the election, though!
(Note that in the above, the sense is the same with or without a comma. The comma can be omitted to avoid interfering with the rhythm of the sentence, but it is not critical to the sense of the adverb.)
Try the substitutions yourself:
- I expected him to lose the election, for all that!
- I expected him to lose the election, nevertheless!
Though here tells us that the speaker either thought her candidate would lose when he won, or that he did lose but that the speaker's reaction is somehow apposite to what is or was expected of her. There are other possibilities as well, but the speaker's expecting is modified by the adverb though. Opposition, or a confounding of expectation, is what though expresses as an adverb.
If a sentence is composed like this, though is a connective preposition:
- Though he tried, he lost.
- He did his best to win, though he lost.
Here, though connects its clause to the main clause, and adds information to it, with the same sense of "notwithstanding" as in the adverbial use, but without acting to modify the action or effect of a verb, as an adverb does.