Learn Russian: Внезапно я осознаю, что это только сон. - Suddenly I realize that this is only a dream.

The infinitive is осознавать (to realize). As you are learning in this lesson, verbs ending in -знавать drop the -ва- suffix in the present tense. So, it conjugates as я осознаю, ты осознаёшь, and so on.

I learned that сон means 'sleep'. Can it also mean 'dream'?

Yes! In Russian, сон is used for both the physical state of 'sleep' and the 'dream' you experience while sleeping. Context tells you which one it is, and here, 'this is only a sleep' wouldn't make sense.

Is осознавать related to the basic verb знать (to know)?

Exactly. They share the core root зна- (know). The prefixes о- and со- build upon it to mean 'to become fully aware of' or 'to realize.' You'll see this same root in other verbs like узнавать (to recognize or to find out), which also drops the -ва- in the present tense.

Do I always need a comma before что in sentences like this?

Yes. Unlike in English, Russian grammar strictly requires a comma before subordinate clauses (a dependent part of a sentence with its own subject and verb). Since что это только сон is a new clause, the comma before что is mandatory.