Я сорвала ветку сирени для мамы и поставила её в вазу.

Because Russian past tense agrees with the speaker’s gender in the singular.

• Я сорвал = a male speaker
• Я сорвала = a female speaker

So this sentence tells us the speaker is female. In the past tense, that is completely normal and very common in Russian.

Сорвала is the past tense of сорвать, a perfective verb. Here it means something like:

• picked
• plucked
• broke off

With flowers, leaves, or small branches, сорвать often suggests taking it off the plant by hand. So сорвала ветку сирени is very natural for picked/broke off a branch of lilac.

It is different from:

• срезать = to cut off, usually with scissors or a knife
• взять = to take

Because ветку is the accusative singular form of ветка.

Here ветку is the direct object of сорвала: the speaker picked a branch.
For most feminine nouns ending in -а, the accusative singular changes to -у:

• ветка → ветку
• мама → маму
• ваза → вазу

So сорвала ветку means picked a branch.

Сирени is genitive singular of сирень.

In ветка сирени, the second noun is in the genitive because Russian often uses this pattern for X of Y:

• ветка сирени = a branch of lilac
• literally, branch of lilac

This is very common in Russian:

• чашка чая = a cup of tea
• кусок хлеба = a piece of bread
• дом брата = the brother’s house / house of the brother

So сирени is there because the sentence means a lilac branch, literally a branch of lilac.

Because the preposition для always takes the genitive case.

• мама → мамы in the genitive singular

So:

• для мамы = for mother / for Mom

This is a rule worth memorizing:

• для + genitive

Examples:

• для друга = for a friend
• для сестры = for a sister
• для детей = for children

Её refers to ветку.

Since ветка is feminine singular, the pronoun is also feminine singular:

• ветка = branch
• её = it referring to that feminine noun

So поставила её в вазу means put it in a vase.

A very important point for English speakers: её can mean her or it, depending on context. Here it clearly means it, because it refers back to ветку.

Yes, it often could, if the meaning is already obvious from context.

For example:

• Я сорвала ветку сирени для мамы и поставила в вазу.

That can still be understood as I picked a branch of lilac for Mom and put it in a vase.

But including её makes the object explicit and sounds perfectly natural. Russian often omits repeated objects when they are clear, but it does not have to.

Because with в, Russian uses different cases depending on whether you mean:

• movement into somewhere → accusative
• location in somewhere → prepositional

Here the branch is being moved into the vase:

• поставила в вазу = put into a vase

Compare:

• ветка стоит в вазе = the branch is standing in the vase
  Here there is no movement, just location, so в вазе is used.

This is a very common Russian pattern:

• класть в сумку = to put into a bag
• лежать в сумке = to lie in a bag

Because the sentence describes two completed actions in sequence:

1. she picked the branch
2. she put it in a vase

Perfective verbs are very common when Russian tells a simple one-time story and focuses on the result.

• сорвать → completed act of picking
• поставить → completed act of placing

If you used the imperfective forms:

• срывала
• ставила

the meaning would change. It could suggest repeated action, process, background description, or a different narrative focus.

So the perfective here is exactly what you would expect in a straightforward completed event.

No, Russian word order is flexible.

The version given is a neutral, natural order:

• Я сорвала ветку сирени для мамы и поставила её в вазу.

But Russian can move parts around for emphasis or style. For example:

• Для мамы я сорвала ветку сирени и поставила её в вазу.
  This emphasizes for Mom.

• Ветку сирени я сорвала для мамы и поставила её в вазу.
  This emphasizes the lilac branch.

So the meaning stays basically the same, but the focus changes.

Russian often does drop subject pronouns when the subject is clear, especially in conversation.

So this is also possible:

• Сорвала ветку сирени для мамы и поставила её в вазу.

That still naturally means I picked a branch of lilac for Mom and put it in a vase, assuming the context makes I clear.

But including я is also normal. It can make the sentence feel a little clearer, a little more explicit, or simply more self-contained.

You do not know from grammar alone, because Russian has no articles like a and the.

So:

• ветку could be a branch or the branch
• вазу could be a vase or the vase

The context decides.

In an isolated sentence, English often uses the most natural option for the situation, for example:

• I picked a branch of lilac for Mom and put it in a vase.

But in a different context, the branch or the vase might be better.

Not exactly. Сирень is lilac, the plant or lilac blossom(s) in a general sense.
Ветка сирени specifically means a branch/sprig of lilac.

That image makes sense culturally too: people often bring home a flowering branch of lilac and put it in a vase.

So the sentence is not about lilac in general, but about one physical branch.

A helpful stress pattern is:

Я сорвалá вéтку сирéни для мáмы и постáвила её в вáзу.

A few useful notes:

• сорвалá — stress on the last syllable
• вéтку — stress on the first syllable
• сирéни — stress on ре
• постáвила — stress on ста
• вáзу — stress on the first syllable
• ё in её is always stressed

Stress matters in Russian, so it is good to learn words together with their stressed syllables.