Kurtka przeciwdeszczowa jest w torbie.

Because w meaning in requires the locative case when it shows location.

• dictionary form: torba = bag
• locative singular: torbie = in the bag

So:

• w torbie = in the bag

If the noun stayed as torba, the sentence would be ungrammatical here.

It is in the nominative case because it is the subject of the sentence — the thing being talked about.

• kurtka = nominative singular
• przeciwdeszczowa matches kurtka in case, number, and gender

So the structure is basically:

• [subject] kurtka przeciwdeszczowa
• [verb] jest
• [place] w torbie

Because the adjective has to agree with the noun it describes.

Kurtka is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• przeciwdeszczowa

Compare:

• kurtka przeciwdeszczowa — feminine
• płaszcz przeciwdeszczowy — masculine
• ubranie przeciwdeszczowe — neuter

Both word orders are possible, but they can sound slightly different.

• przeciwdeszczowa kurtka = a normal descriptive order, very natural
• kurtka przeciwdeszczowa = also natural, and often used when naming a type/category of item

With items of clothing, equipment, and official-style labels, Polish often puts this kind of adjective after the noun:

• kurtka przeciwdeszczowa
• buty sportowe
• koszula nocna

So the sentence is natural as written.

It is built from:

• przeciw- = against
• deszcz = rain

So literally it means something like against-rain or rain-protective.

In natural English, that usually corresponds to:

• rain jacket
• rainproof jacket
• raincoat-type jacket, depending on context

Yes, in a normal full sentence, jest is needed.

Polish usually does not drop the present-tense form of to be in standard sentences like this.

So:

• Kurtka przeciwdeszczowa jest w torbie. = normal full sentence

If you say only:

• Kurtka przeciwdeszczowa w torbie.

it sounds incomplete, like a note, label, or very casual shorthand rather than a standard sentence.

Yes. Polish word order is fairly flexible, and changing it changes the focus more than the core meaning.

For example:

• Kurtka przeciwdeszczowa jest w torbie. — neutral
• W torbie jest kurtka przeciwdeszczowa. — focuses more on where it is
• Jest w torbie kurtka przeciwdeszczowa. — possible, but less neutral and more marked

So the original sentence is a straightforward, neutral option.

Because Polish has no articles.

So kurtka can mean:

• a jacket
• the jacket

and torbie can mean:

• in a bag
• in the bag

The exact meaning depends on context, not on separate words like a or the.

No. W torbie means in the bag as a location.

If you want movement into the bag, Polish normally uses do + genitive:

• do torby = into the bag

For example:

• Kurtka jest w torbie. — The jacket is in the bag.
• Wkładam kurtkę do torby. — I am putting the jacket into the bag.

So:

• w torbie = location
• do torby = direction/movement into

A rough pronunciation guide is:

• pshe-cheev-desh-CHO-va

A few helpful points:

• prz sounds roughly like psh/zh blended together
• cz sounds like ch in chop
• w in Polish sounds like English v
• the stress is usually on the second-to-last syllable

So the stress is on:

• przeciwdeszCZOwa

When w means in and shows a location, yes, it usually takes the locative:

• w torbie — in the bag
• w domu — in the house
• w szkole — at school / in school

That is the pattern used in this sentence. For learners, the safest rule here is:

• w
  • place/location = usually locative