На лестнице её подол зацепился за каблук, но она сразу это заметила.

Because на often means location when it is followed by the prepositional case.

• лестница = staircase, stairs
• на лестнице = on the stairs / on the staircase

The form лестнице is the prepositional singular of лестница.

So this part sets the scene: the event happened while she was on the stairs.

Here её means her in the possessive sense:

• её подол = her hem

A useful thing to know is that её does not change form. It can mean:

• her
• hers
• sometimes her as an object, depending on context

In this sentence, it is clearly possessive because it comes before a noun: её подол.

Подол means the hem or lower edge of a skirt or dress.

In this sentence, it most naturally means something like:

• the hem of her dress
• the hem of her skirt

So её подол is not a whole item of clothing, but specifically the part at the bottom.

Зацепиться means to catch on, to get snagged, or to get hooked on something.

So:

• подол зацепился за каблук = the hem caught on a heel

The past tense form зацепился is masculine singular because it agrees with подол, which is a masculine noun.

Compare:

• подол зацепился — masculine
• юбка зацепилась — feminine

The -ся makes the verb reflexive, but in many verbs that does not mean a literal English self meaning.

Here зацепиться is just the normal verb meaning:

• to catch on something
• to get snagged

So зацепился does not mean that the hem actively hooked itself in a weird literal sense. It is simply the standard Russian way to express that something got caught.

With зацепиться, Russian normally uses the pattern:

• зацепиться за + accusative

So:

• за каблук = on a heel / against a heel / caught on a heel

Here каблук is in the accusative singular, and for this masculine inanimate noun, the accusative looks the same as the nominative:

• nominative: каблук
• accusative: каблук

This is a fixed and very common pattern:

• зацепиться за гвоздь = catch on a nail
• зацепиться за ветку = catch on a branch

It is usually understood to be her heel, more specifically the heel of her shoe.

Russian often leaves this kind of thing unstated when it is obvious from context. Since no other person is mentioned, readers naturally understand:

• её подол зацепился за каблук = her hem caught on her heel

The singular каблук is also natural, because the hem caught on one heel, not both.

Она is included to make the subject clear right away.

In the first clause, the grammatical subject is подол. In the second clause, the subject changes to она. Russian often states the pronoun when there is a change of subject or when clarity is important.

Without она, the sentence would be less clear at first glance, because the reader has just been following подол as the subject.

So:

• но она сразу это заметила = but she noticed it immediately

The она helps smoothly shift the focus from the hem to the woman.

Because это refers to the whole situation, not just to the noun подол.

She noticed that this had happened:

• that the hem had caught
• that something was wrong with the hem
• that it had snagged on her heel

If Russian used его, that would most likely refer specifically to подол as a masculine noun:

• его = him / it, masculine

But that is not the main idea here. The idea is that she noticed the incident, so это is the natural choice.

Сразу means immediately, right away, or at once.

In this sentence:

• она сразу это заметила = she noticed it immediately

Russian word order is flexible, so сразу can move around depending on emphasis, for example:

• она сразу это заметила
• она это сразу заметила
• сразу она это заметила — possible, but more marked

The version in the sentence is very natural and neutral.

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

• зацепился ends in -ся after the masculine past ending, because the subject is подол:
  • подол is masculine
• заметила has the feminine ending -а, because the subject is она:
  • она refers to a woman

So the sentence shows two different subjects:

• подол зацепился
• она заметила

This is a very important feature of Russian past tense.

Both verbs describe completed single events.

• зацепился = got caught
• заметила = noticed

Perfective aspect is natural because the sentence tells us about two specific things that happened once, one after the other.

If you used imperfective forms, the meaning would change. For example, imperfective might suggest:

• an ongoing process
• repeated action
• background description

But here the sentence is about a quick sequence of completed events, so perfective is exactly right.

It is very natural Russian word order.

Structure:

• На лестнице — setting
• её подол зацепился за каблук — what happened
• но она сразу это заметила — contrast / follow-up

Russian word order is flexible, but this version sounds smooth and idiomatic. It gives the location first, then the event, then the reaction.

A more literal structure closer to English is possible in some contexts, but this sentence already sounds completely normal.

Because но means but, and it joins two clauses:

• её подол зацепился за каблук
• она сразу это заметила

In Russian, when но connects two full clauses, a comma is normally required.

So the comma here is standard punctuation, just like in English:

• ..., but she noticed it immediately.