Learn Russian: Чтобы сад не начал гнить, убирайте листья как можно чаще. - To prevent the garden from starting to rot, remove the leaves as often as possible.

The phrase как можно literally translates to "how it is possible". When you follow it with a comparative adjective or adverb, like the adverb чаще (more often), it creates the structure "as [adverb/adjective] as possible". Therefore, как можно чаще means "as often as possible".

Why is the past tense verb начал used after чтобы when we are talking about preventing something in the future?

When the subject of the main clause (the implied "you" doing the cleaning) is different from the subject of the чтобы clause (сад), Russian strictly requires a past tense verb in the subordinate clause. Even though it describes a general or future prevention, чтобы сад не начал is the mandatory structure for "so the garden won't start".

Why does the imperative убирайте use the imperfective aspect?

The imperfective aspect is used here because the action is repeated and regular. The phrase как можно чаще (as often as possible) explicitly tells us this is an ongoing habit rather than a single, one-time completed action, so the imperfective убирайте is required.

Why must гнить (to rot) be in the imperfective aspect after начал?

The verb начал is a phase verb, which is a verb indicating the start, continuation, or end of an action. In Russian, phase verbs must always be followed by an imperfective infinitive because the secondary action (the rotting) is viewed as an ongoing process rather than a single completed event.