Punctuation
What to master
Punctuation depends on sentence structure. You need commas with homogeneous members, detached definitions and circumstances, introductory words, appeals, and subordinate clauses; dashes between subject and predicate and in elliptical sentences; colons and dashes in complex non-conjunctive sentences; and punctuation in direct speech and quotations. Know when a comparative phrase with как takes a comma and when it does not.
Traps
Do not put a comma between subject and predicate unless there is a clear dash rule. Avoid separating a subordinate clause from its main clause incorrectly when a compound conjunction is present. Introductory words must be distinguished from members of the sentence: «Он, кажется, устал» vs «Он кажется усталым». A participial phrase after the noun is normally detached, but not always. With как, the comma depends on comparison versus identification.
Worked example
In «Когда я вернулся, она уже ушла», the comma marks the boundary between the subordinate and main clauses. In «Он вернулся, чтобы помочь», чтобы introduces a purpose clause and is preceded by a comma. Rule of thumb: mark bases, then boundaries, then insert punctuation only at justified boundaries.
Rule of thumb
Ask whether the punctuation marks a syntactic boundary; if not, a comma is probably unnecessary.