Learn Russian: Погода странная: солнце то светит, то исчезает. - The weather is strange: the sun now shines, now disappears.

The repeated conjunction то ..., то ... shows that two actions are alternating back and forth. Instead of happening at the same time, the sun is shining one moment, and disappearing the next. It translates well to "now ..., now ..." or "one moment ..., the next ...".

Why are the verbs светит and исчезает both in the imperfective aspect?

Because the то ..., то ... structure describes a repeating, cyclical pattern (shining, then disappearing, then shining again). Repeated actions in Russian naturally require imperfective verbs.

Do I always need a comma before the second то?

Yes. In Russian, when you use repeating conjunctions like то ..., то ... to connect actions, a comma is strictly required before the second conjunction.

Why is there a colon after странная instead of a period or comma?

The colon is used because the second half of the sentence (солнце то светит, то исчезает) directly explains the statement in the first half (Погода странная). It functions a bit like the English phrases "namely" or "because".