Когда началась метель, я поднял капюшон.

Because the subject is метель, and метель is a feminine singular noun.

In the Russian past tense, the verb agrees with the subject in gender and number:

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

So with метель, you need началась.

That -сь shows that the verb is reflexive: начаться.

Russian often uses:

• начать = to start something
• начаться = to begin, to start

Compare:

• Я начал урок. = I started the lesson.
• Урок начался. = The lesson began.

In your sentence, the blizzard is not starting something else; the blizzard itself begins, so Russian uses начаться.

Both word orders are possible.

• Началась метель sounds very natural in narration and often puts the focus on the event starting
• Метель началась is also correct, but feels a bit more neutral or contrastive depending on context

Russian word order is more flexible than English. In this sentence, verb-first sounds very normal.

Началась is perfective because it refers to a single completed starting point: the moment the blizzard began.

That fits the meaning of when the blizzard started, I...

Начиналась is imperfective and would usually suggest:

• a process
• background description
• repeated or less clearly bounded action

So in this sentence, началась is the natural choice.

Поднял is:

• past tense
• masculine singular
• perfective

So it suggests the speaker is male.

If the speaker were female, it would be:

• я подняла капюшон

The perfective aspect is used because this is a single completed action: the speaker pulled up the hood once.

Капюшон is in the accusative case because it is the direct object of поднял.

However, for many inanimate masculine singular nouns, the accusative looks exactly like the nominative.

So:

• nominative: капюшон
• accusative: капюшон

That is why there is no visible ending change here.

Russian often leaves out possessive words like мой, твой, его when the owner is obvious from the context.

Here, if I raised a hood, it is naturally understood to mean my hood.

So:

• я поднял капюшон = I pulled up my hood

Adding мой is possible, but usually unnecessary unless you want emphasis or contrast:

• я поднял мой капюшон would sound unusual in most contexts
• more natural for emphasis would be я поднял свой капюшон

But in this sentence, plain капюшон is the most natural choice.

Because Когда началась метель is a subordinate clause, and Russian separates subordinate clauses with commas.

So the structure is:

• Когда началась метель, = when the blizzard began
• я поднял капюшон. = I pulled up my hood

Russian uses this comma even where English learners might hesitate.

Yes, absolutely.

Both are correct:

• Когда началась метель, я поднял капюшон.
• Я поднял капюшон, когда началась метель.

The difference is mostly in focus:

• starting with Когда началась метель sets the scene first
• starting with Я поднял капюшон puts the main action first

Here it means when, in the sense of at the moment that.

That is because the verb началась marks a specific point in time: the blizzard began.

So the sentence means:

• at the moment the blizzard started, I pulled up my hood

If you wanted more of a while meaning, Russian would usually use a different structure and often an imperfective verb, depending on the exact idea.