После тренировки я сняла мокрую толстовку и долго сидела в раздевалке.

Because после requires the genitive case.

The dictionary form is тренировка, but after после it changes to тренировки:

• тренировка = workout, training session
• после тренировки = after the workout

This is a very common pattern:

• после урока = after the lesson
• после работы = after work
• после фильма = after the film

In the past tense, Russian verbs show gender in the singular.

So:

• снял / сидел = masculine speaker or subject
• сняла / сидела = feminine speaker or subject
• сняло / сидело = neuter
• сняли / сидели = plural

Here я сняла and я сидела tell you the speaker is female.

Sometimes yes, but here я is especially useful.

In the past tense, Russian verbs do not show person clearly:

• сняла can mean I took off or she took off, depending on context
• сидела can mean I sat or she sat

So я removes ambiguity. Without it, the sentence could sound incomplete unless the context already makes the subject obvious.

Because толстовку is the direct object of сняла, so it is in the accusative case.

Dictionary form:

• мокрая толстовка = a wet sweatshirt/hoodie

In the sentence:

• сняла мокрую толстовку = took off a wet sweatshirt/hoodie

Both the adjective and noun change:

• мокрая → мокрую
• толстовка → толстовку

This is the normal accusative pattern for feminine singular nouns and adjectives.

Here сняла means took off or removed.

The verb is снять. In this context, it is used for removing clothes:

• снять куртку = take off a jacket
• снять обувь = take off shoes

This verb can have other meanings in other contexts, such as rent, take down, or film, but here the clothing context makes take off the correct meaning.

Because the sentence combines:

• one completed action
• one ongoing/durative action

сняла is perfective from снять. It presents taking off the sweatshirt as a single completed event.

сидела is imperfective from сидеть. It describes a state or activity lasting for some time.

So the sentence feels like:

• first, she took off the sweatshirt
• then she sat there for a while

This is a very natural aspect choice in Russian.

Because в meaning in for location usually takes the prepositional case.

Dictionary form:

• раздевалка = changing room, locker room

After в for location:

• в раздевалке = in the changing room / in the locker room

Compare:

• в школе = in school
• в комнате = in the room
• в раздевалке = in the locker room

If в showed motion into a place, Russian would often use the accusative instead:

• в раздевалку = into the locker room

But here the meaning is location, not movement.

Толстовка usually means a casual warm top like a hoodie, sweatshirt, or similar item.

In everyday Russian, it does not always strictly mean a hooded sweatshirt. Depending on the speaker and region, it may refer to:

• a hoodie
• a sweatshirt
• a zip-up sweatshirt

So English translation depends on context.

Раздевалка is a place where people change clothes. Depending on context, it can be:

• changing room
• locker room
• dressing room

In a sports context like this, locker room or changing room is usually best.

Долго is an adverb meaning for a long time or a long time.

It modifies сидела:

• долго сидела = sat for a long time

It tells you the sitting lasted for some time, rather than being a brief moment.

No, Russian word order is fairly flexible.

This version is natural:

• После тренировки я сняла мокрую толстовку и долго сидела в раздевалке.

But other orders are possible, depending on emphasis:

• Я после тренировки сняла мокрую толстовку и долго сидела в раздевалке.
• Мокрую толстовку я сняла после тренировки и долго сидела в раздевалке.

The original order sounds neutral and smooth: it starts with the time setting after the workout, then gives the actions.

Putting после тренировки first sets the scene immediately.

Russian often places time expressions near the beginning when they provide the background for the whole sentence. It is similar to English:

• After the workout, I took off my wet sweatshirt...

It tells the listener when everything happened before moving on to the main actions.

The main stress in each word is:

• послЕ
• тренирОвки
• снялА
• мОкрую
• толстОвку
• дОлго
• сидЕла
• в раздевАлке

A natural reading is: ПослЕ тренирОвки я снялА мОкрую толстОвку и дОлго сидЕла в раздевАлке.

For an English speaker, one useful thing to notice is that unstressed vowels are reduced in Russian, so the spelling and pronunciation are not always identical.