Because w here means in and refers to a location, so it requires the locative case.
So both the noun and any matching word like ten/ta/to change form.
Because koszula is in the locative singular after w.
This is a normal pattern for many feminine nouns ending in -a:
So koszuli does not mean a different word; it is just the case form of koszula.
Because the verb brakować / brakuje usually takes the thing that is missing in the genitive case.
So:
Compare:
A useful way to think about brakuje is there is a lack of... or ...is missing.
So:
In natural English, we usually say:
Polish often structures this idea differently from English.
Yes, absolutely. That is natural Polish too.
The difference is roughly this:
So brakuje often gives a stronger sense that something expected is missing.
A often means and, but in many sentences it has a slight contrastive feel, like:
Here it links two related observations:
So a is very natural here because the second part adds a contrasting parallel detail.
Yes, zamek has several meanings, including:
Polish uses the same word for all of these, and the context tells you which meaning is intended.
Here, because we have w kurtce and we are talking about clothing, zamek clearly means zipper.
For the same reason as w tej koszuli: w with a location uses the locative.
This particular change is common for feminine nouns ending in -ka.
Yes. Polish word order is fairly flexible.
For example, these are all possible:
The original version sounds natural because it introduces each clothing item first, then says what the problem is.
Because Polish has no articles like English a/an/the.
So Polish simply says:
Whether English needs a or the depends on context and translation, not on a separate Polish word.