Когда началась метель, снежинки быстро покрыли скамейку и перила.

Because Когда началась метель is a subordinate clause introduced by когда (when). In Russian, a subordinate clause is normally separated from the main clause with a comma.

So the sentence is divided like this:

• Когда началась метель, = the when-clause
• снежинки быстро покрыли скамейку и перила. = the main clause

Because the subject of that verb is метель, which is a feminine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

Since метель is feminine singular, you get началась.

The dictionary form is начаться, which means to begin / to start in the sense of to begin on its own or to start happening.

So:

• начаться = to begin, to start
• началась = began / started

The -ся / -сь ending is the reflexive marker. In this sentence, метель началась means the snowstorm started.

Because some feminine nouns in Russian belong to the third declension and end in -ь.

So метель is:

• feminine
• singular
• nominative here

This is just a noun pattern you need to learn. Other similar feminine nouns ending in -ь include:

• дверь = door
• ночь = night
• тетрадь = notebook

So even though метель does not end in -а or -я, it is still feminine.

Because снежинки is the subject of the main clause, and it is plural.

• снежинка = snowflake
• снежинки = snowflakes

The past tense verb has to agree in number with the subject:

• снежинка покрыла = one snowflake covered
• снежинки покрыли = snowflakes covered

So покрыли is the plural past-tense form.

Because покрыли is perfective, and it presents the action as a completed event.

• покрыть → perfective
• покрывать → imperfective

In this sentence, the idea is that once the blizzard started, the snowflakes quickly covered the bench and the railing. That sounds like a completed result, so покрыли fits well.

If you used покрывали, it would sound more like an ongoing or repeated process:

• снежинки быстро покрывали... = the snowflakes were quickly covering...

That is possible in some contexts, but покрыли is more natural here.

Because it is the direct object, so it goes into the accusative case.

Dictionary form:

• скамейка = bench

In the accusative singular:

• скамейку

So:

• покрыли что? = covered what?
• скамейку

This is a very common change for feminine nouns ending in -а:

• книга → книгу
• машина → машину
• скамейка → скамейку

Because перила is a plural-only noun, and here it is inanimate. For inanimate plural nouns, the accusative is usually the same as the nominative.

So:

• nominative: перила
• accusative: перила

That is why you see:

• покрыли скамейку и перила

Also, перила is normally used only in the plural, even when English might say railing in the singular sense.

Быстро is an adverb meaning quickly, and Russian word order is fairly flexible. Putting быстро before the verb is very normal.

• снежинки быстро покрыли... = neutral, natural
• снежинки покрыли... быстро = possible, but different emphasis
• быстро снежинки покрыли... = possible, but more marked or literary

So the sentence uses the most straightforward order.

Yes. Both are grammatically correct.

• Когда началась метель = neutral, very natural
• Когда метель началась = also correct, but with slightly different emphasis

Russian word order can shift focus. Putting the verb first in this clause gives a smooth narrative flow: When the blizzard started...

So the version in the sentence is probably chosen because it sounds especially natural in storytelling.

Both are possible, but they create slightly different imagery.

• снежинки = snowflakes, more visual and specific
• снег = snow, more general

Using снежинки makes the scene feel more vivid, as if you can picture the individual flakes covering the bench and railing.

So this is partly a vocabulary choice, not just a grammar choice.

Here когда is a subordinating conjunction meaning when.

It introduces the time clause:

• Когда началась метель = When the blizzard began

It is not asking a question here. It is linking two parts of the sentence together.

Compare:

• Когда началась метель? = When did the blizzard begin? → question word
• Когда началась метель, ... = When the blizzard began, ... → conjunction introducing a clause