Learn Russian: Мы гуляем, когда погода хорошая. - We go for a walk when the weather is good.

In Russian, the verb гулять specifically means "to stroll" or "to go for a walk." You don't need to combine it with a verb of motion like идти (to go) to express this idea.

Is the comma before когда required?

Yes! Unlike English, Russian punctuation rules require you to always put a comma before conjunctions like когда (when), что (that), and потому что (because) when they introduce a new clause (a part of the sentence that has its own subject and verb).

Why is there no word for "is" in погода хорошая?

In the present tense, Russian almost always leaves out the verb "to be" (am, is, are). So, погода хорошая literally translates to "weather good," but it functions as a complete sentence meaning "the weather is good."

Could I change the word order and say когда хорошая погода instead?

Yes, but the feeling shifts slightly. Погода хорошая acts like a full statement ("the weather is good"). Хорошая погода acts more like a descriptive phrase ("good weather"). Both work here and mean practically the same thing, but the current order puts more emphasis on the fact that the weather is good.