Learn Dutch: De tegenstander praat alsof hij echt is. - The opponent talks as if he is real.

The word alsof (as if) is a subordinating conjunction. This means it introduces a dependent clause, which forces the conjugated verb (is) all the way to the end of that clause.

Since alsof makes a hypothetical comparison, why use the present tense is instead of the past tense was?

While you could use was to strongly emphasize that the opponent isn't actually real (similar to saying 'as if he were real' in English), Dutch often just matches the tense of the main clause. Since praat (talks) is in the present tense, using is sounds completely natural and common here.

What are the individual parts that make up the word tegenstander?

It is built from tegen (against) and stander (someone who stands). Literally, it means someone who stands against you, making it a very logical way to remember the Dutch word for opponent or adversary.

I usually see echt used to mean 'really'. Is it being used the same way here?

Not quite. While echt often works as an adverb to add emphasis (like saying a game is 'really good'), here it is an adjective meaning 'real' or 'genuine'. The sentence means he talks like a real person, not that he is 'really' doing something.