Learn Russian: Озеро застыло, как будто это хрустальное зеркало. - The lake froze, as if it were a crystal mirror.

Unlike English, which uses the past subjunctive ("were") for hypotheticals, Russian simply uses the normal indicative tense that fits the scene. Since the lake is being compared to what a crystal mirror is right now, Russian uses the present tense. Because the present tense of "to be" is dropped, we just see это хрустальное зеркало.

Could I use словно or точно instead of как будто in this sentence?

Absolutely. Озеро застыло, словно это хрустальное зеркало works perfectly and means the exact same thing. Как будто is the most common and neutral choice, while словно sounds a bit more literary and poetic, which actually suits this beautiful winter imagery very well.

Why is the word это used here? Couldn't we just say как будто хрустальное зеркало?

You definitely can drop it! If you drop это, it becomes a simple phrase comparison ("as if a crystal mirror") rather than a full subordinate clause ("as if it is a crystal mirror"). Including это simply gives the sentence a slightly more deliberate rhythm, drawing out the imagery of the mirror.

I usually use замёрзло for "froze". Why does this sentence use застыло?