После тренировки сестра вернулась в раздевалку и забыла ключ от шкафчика.

Because после always requires the genitive case in Russian.

• тренировка = training / practice / workout
• Genitive singular: тренировки

So:

• после тренировки = after the training / after practice

This is a very common pattern:

• после урока = after the lesson
• после работы = after work
• после школы = after school

So the learner should remember: после + genitive.

Сестра is the subject of the sentence, so it is in the nominative case.

Here, сестра means sister, and it is the one doing the actions:

• she returned
• she forgot / left behind the key

So nominative is used because it answers who did it?
→ сестра

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

The subject is сестра, which is feminine singular, so the verb must also be feminine singular:

• вернулся = he returned
• вернулась = she returned
• вернулось = it returned
• вернулись = they returned

So сестра вернулась literally means the sister returned, with the verb showing that the subject is feminine.

The verb is вернуться, which is the reflexive form of вернуть.

• вернуть = to return something
• вернуться = to return, to come back

So:

• Она вернула книгу = She returned the book.
• Она вернулась домой = She returned home / came back home.

In your sentence, вернулась means came back / returned.

The -ся ending often changes a verb into something like oneself, back, or an intransitive meaning, depending on the verb. With вернуться, it simply means to come back.

Because в can take either the accusative or the prepositional, depending on the meaning.

Here the sentence expresses motion toward a place:

• вернулась в раздевалку = returned to the locker room / changing room

So Russian uses в + accusative.

Compare:

• в раздевалку = into / to the locker room (direction, movement)
• в раздевалке = in the locker room (location)

Examples:

• Она вошла в раздевалку. = She went into the locker room.
• Она была в раздевалке. = She was in the locker room.

Раздевалка means a changing room, locker room, or cloakroom, depending on context.

In this sentence, because there is also шкафчик (locker), locker room or changing room is the most natural understanding.

It comes from the verb раздеваться, meaning to get undressed.

So learners can think of раздевалка as the place where people change clothes.

Yes, от often means from, but in Russian it is also used in the expression:

• ключ от ... = key to / key for ...

So:

• ключ от двери = key to the door
• ключ от машины = car key
• ключ от квартиры = apartment key
• ключ от шкафчика = key to the locker

This is just the normal Russian pattern. Even though it looks like key from the locker if translated word-for-word, the real meaning is locker key or key to the locker.

Because от requires the genitive case.

The base word is:

• шкафчик = small cabinet / locker

After от, it becomes:

• от шкафчика

So this part works exactly like many other Russian prepositions that require a specific case.

Also, шкафчик is a very normal word for a gym or school locker.

In this sentence, it most naturally means she left the key behind.

Russian забыть can mean both:

1. to forget in the mental sense
2. to forget something somewhere / leave it behind

With a concrete object like ключ, especially in a context like going back to the locker room, the second meaning is very natural:

• забыла ключ = forgot the key / left the key behind

So in English, depending on context, you might translate it as:

• forgot the locker key
• left the locker key behind

Because the sentence describes completed actions in sequence.

• вернулась = she returned
• забыла = she forgot / left behind

These are seen as whole events that happened once.

Russian often uses the perfective aspect for this kind of narrative:

• one completed action happened,
• then another completed action happened.

If you used imperfective forms, the meaning would change:

• возвращалась = was returning / used to return
• забывала = was forgetting / used to forget

Those would sound wrong or at least very different here, because the sentence is about a specific finished situation.

Yes. Russian word order is fairly flexible, because case endings show the grammatical roles.

The given sentence is natural:

• После тренировки сестра вернулась в раздевалку и забыла ключ от шкафчика.

But you could also say:

• Сестра после тренировки вернулась в раздевалку и забыла ключ от шкафчика.

Both are grammatical. The original version puts После тренировки first, which gives the time setting right away: After practice...

That is a very common Russian style.

Yes. И simply means and, and here it links two actions done by the same subject:

• сестра вернулась
• (сестра) забыла

In Russian, once the subject is clear, it does not need to be repeated.

So the full logic is:

• After practice, the sister returned to the locker room and forgot the locker key.

Russian does this very naturally without repeating сестра a second time.

Stress is important in Russian because it is not always predictable. In this sentence, the usual stresses are:

• после
• тренировки
• сестра
• вернулась
• в раздевалку
• и
• забыла
• ключ
• от
• шкафчика

A few especially useful ones to remember:

• тренировка
• вернуться
• забыть
• раздевалка

Stress is worth learning together with the word, because it can move in different forms.