Learn Hardcore Russian: Она прячет маленькую записку в карман куртки. - She hides a small note in the pocket of her jacket.

Записка is feminine, singular.
As a direct object, it must be in the accusative case.
Feminine singular adjectives and nouns change their endings in the accusative:
- маленькая → маленькую (fem. nom. → fem. acc.)
- записка → записку (fem. nom. → fem. acc.)

So the phrase маленькую записку is in the accusative case and shows what she is hiding.

Why does записка become записку?

Записка is a regular first-declension feminine noun:

nominative singular: записка (subject form)
accusative singular: записку

In this sentence, записку is the thing being hidden, so it must be in the accusative.
That is why the ending changes from -а to -у.

Why is it в карман and not в кармане?

The preposition в can take either the accusative or the prepositional case, with different meanings:

в + accusative = direction, movement into something (where to?)
- в карман → into the pocket
в + prepositional = location, being in something (where?)
- в кармане → in the pocket

Here she is putting the note into the pocket, so there is movement.
Therefore Russian uses в карман (accusative), not в кармане.

Why is куртки used and not куртка or куртке?

Куртки here is the genitive singular of куртка.

The phrase карман куртки literally means the pocket of the jacket.
In Russian, possession or belonging is usually shown with the genitive case:

карман куртки → the pocket of the jacket
дом брата → the house of (my) brother

So:

карман is in the accusative (as part of в карман – direction),
куртки is in the genitive (showing whose pocket it is).

Why is it карман куртки and not куртки карман, like English word order?

In Russian, when one noun possesses another, the typical pattern is:

[thing possessed] + [owner in genitive]

So:

карман куртки = the pocket of the jacket
страницы книги = the pages of the book

You almost never reverse it to куртки карман in standard Russian.
The natural, neutral order is карман куртки.

What is the aspect and tense of прячет, and how would I say “will hide” or “hid”?

Прячет is:

the 3rd person singular form (he/she/it)
of the verb прятать
in the present tense
imperfective aspect (focus on process or repeated action)

Depending on context, она прячет can mean:

she is hiding (right now), or
she hides (generally, habitually)

For a single, completed act (perfective aspect), you use спрятать:

Future (single event):
- Она спрячет маленькую записку в карман куртки. → She will hide the note in the pocket.
Past (completed event):
- Она спрятала маленькую записку в карман куртки. → She hid the note in the pocket.

Imperfective past (ongoing or repeated in the past) would be:

Она прятала маленькую записку... → She was hiding / used to hide the note…

Could I use скрывает instead of прячет?

You sometimes can, but they are not full synonyms:

прятать / спрятать focuses on physically putting something out of sight, often in a specific place:
- hide a note in a pocket, hide money under the mattress, hide behind a tree.
скрывать / скрыть is more about concealing, keeping secret, often in an abstract sense:
- conceal feelings, hide information, keep something secret.

In this specific sentence, она прячет маленькую записку в карман куртки sounds completely natural.
Она скрывает маленькую записку в карман куртки sounds unusual and too literal, as if you're forcing a more abstract verb into a very physical context.

Why not use прячется (reflexive) here?

Прячется is the reflexive form of прятать:

она прячет = she hides something (takes a direct object)
она прячется = she hides herself

In this sentence, she is hiding a note, not herself, so the non‑reflexive form is needed:

Она прячет маленькую записку... = She hides a small note…
Она прячется в шкафу. = She is hiding in the wardrobe.

Could I drop the pronoun and just say Прячет маленькую записку в карман куртки?

You can, but it is less neutral and depends on context.

In Russian, subject pronouns (я, ты, он, она…) are usually kept in neutral written and spoken language:

Она прячет маленькую записку... is the normal form.

You might omit она if:

the subject is already very clear from the previous sentence, and
you want a more clipped, narrative, or stylistic effect.

For a typical learner sentence, keep она.

How flexible is the word order? Can I move маленькую записку or в карман куртки?

Russian word order is relatively flexible, because grammar is shown by endings, not position.
All of these are grammatically correct, but differ in emphasis:

Она прячет маленькую записку в карман куртки.
(neutral: she hides a small note into the pocket of the jacket.)
Она прячет в карман куртки маленькую записку.
(slight focus on where she hides it: into the pocket.)
Маленькую записку она прячет в карман куртки.
(emphasis on the small note specifically.)

For learners, the original order is the safest and most natural.

How do you pronounce the tricky parts of this sentence?

Approximate pronunciation (stressed syllables in caps):

Она → a-NA
прячет → PRYA-chet
- рь is soft; я = ya; ч = ch.
маленькую → MA-len’-ku-yu
- stress usually on the first syllable: МА‑лень‑кую.
записку → za-PIS-ku
в → v (very short, often almost attached to the next word)
карман → kar-MAN
куртки → KURT-ki
- рт is pronounced together: rt; ки with a soft k.

Russian stress is important: changing it can change the word or sound unnatural.

Is записка already “a little note”? Why say маленькую записку?

Historically, записка is related to запись (record, entry), and in many contexts it does mean a short written note.

However, маленькую записку adds:

a clear idea of physical smallness (a small piece of paper, not a bigger letter),
extra emphasis, a bit like a tiny note or a little note in English.

So записка alone is just a note; маленькая записка is explicitly small in size.