Я спрячу ключ в карман куртки.

Спрячу is the 1st person singular future form of the verb спрятать.

• спрятать = to hide, to put away and have it hidden
• я спрячу = I will hide

This verb is perfective, which means it presents the action as a single completed result. So Я спрячу ключ... means I will hide the key with the idea that the key will end up hidden.

In Russian, perfective verbs form the future with a single word, not with a separate word like will.

So:

• я спрячу = I will hide
• literally, it is just one verb form

Compare:

• прятать / прячу = to hide, to be hiding, to hide regularly or as a process
• спрятать / спрячу = to hide completely, to put away successfully

So Russian expresses this difference through aspect, not with a separate future auxiliary like English.

This is a very common and important question.

• прячу comes from прятать and usually means I am hiding, I hide, or I hide regularly
• спрячу comes from спрятать and means I will hide / I’ll put it away so it is hidden

So:

• Я прячу ключ = I am hiding the key / I hide the key
• Я спрячу ключ = I will hide the key

Very roughly:

• прятать = process / repeated action / ongoing action
• спрятать = completed result

Because ключ is the direct object of the verb, so it is in the accusative case.

But for an inanimate masculine noun like ключ, the accusative singular looks exactly the same as the nominative singular:

• nominative: ключ
• accusative: ключ

So even though it is the object, the form does not change.

If the noun were animate, you would often see a different form.

Because this sentence shows movement/direction into something, not location.

With в:

• в + accusative = into
• в + prepositional = in / inside

So:

• в карман = into the pocket
• в кармане = in the pocket

Compare:

• Я спрячу ключ в карман куртки. = I will hide the key into the jacket pocket.
• Ключ в кармане куртки. = The key is in the jacket pocket.

This is one of the most important uses of Russian cases after prepositions.

Куртки is the genitive singular of куртка.

Here it means of the jacket, so:

• карман куртки = the pocket of the jacket
• more naturally in English: the jacket pocket or the pocket of the jacket

This is a very common Russian pattern:

• дверь дома = the door of the house
• центр города = the city center
• карман куртки = the jacket pocket

So куртки is not the object of the verb. It depends on карман and tells you whose/which pocket.

Yes, very often.

Russian usually does not need the subject pronoun when the verb ending already makes the subject clear.

• Спрячу ключ в карман куртки. = I’ll hide the key in the jacket pocket.
• Я спрячу ключ в карман куртки. = I’ll hide the key in the jacket pocket.

Including Я can add:

• emphasis
• contrast
• clarity in context

For example:

• Я спрячу ключ, а ты закрой дверь. = I’ll hide the key, and you close the door.

So in a neutral sentence, dropping Я is often completely natural.

No, Russian word order is fairly flexible.

Я спрячу ключ в карман куртки is a neutral, natural order.

But you could change the order to shift emphasis:

• Ключ я спрячу в карман куртки.
  Emphasis on the key
• В карман куртки я спрячу ключ.
  Emphasis on where it will be hidden
• Я в карман куртки спрячу ключ.
  Also possible, depending on context and style

The basic meaning stays the same, but the focus changes.

Yes, that is a very literal breakdown:

• в = into / in
• карман = pocket
• куртки = of the jacket

But in natural English, you would often say:

• in the jacket pocket
• into the pocket of the jacket

Because the Russian sentence uses в + accusative, the idea is specifically into the pocket, not just in the pocket as a static location.

The main stress is:

• Я спрЯчу ключ в кармАн кУртки.

A rough pronunciation guide:

• Я = ya
• спрячу = SPRYA-chu
• ключ = klyuch
• в карман = f kar-MAN
• куртки = KOORT-kee

A few useful notes:

• в often sounds like f before a voiceless consonant, so в карман may sound close to f karmán
• ключ has the кл cluster plus ю, which can be tricky for English speakers
• спрячу begins with a consonant cluster that may also take practice

Not in this sentence, if you want the same grammar.

• Я спрячу ключ в карман куртки. = I will hide the key into the jacket pocket.
• в кармане куртки means in the jacket pocket, which describes location, not direction.

So в кармане куртки would fit better in a sentence like:

• Ключ лежит в кармане куртки. = The key is in the jacket pocket.

With спрячу, Russian normally uses the form showing destination: в карман.

Here are the base forms:

• я = I
• спрячу → dictionary form спрятать
• ключ = key
• в = in, into
• карман = pocket
• куртки → dictionary form куртка = jacket

This is a useful habit when reading Russian: identify the changed forms and trace them back to the dictionary form.