W tym semestrze mamy mało czasu, bo co tydzień jest kartkówka.

Because w can take different cases depending on meaning.

• w + locative = in / during / inside
• w + accusative = into (movement toward the inside of something)

Here, W tym semestrze means in this semester / during this semester, so we use the locative.

Forms:

• ten semestr = nominative
• w tym semestrze = locative

So the ending changes because the case changes.

Because tym is the locative singular form of ten for a masculine/neuter noun.

The noun semestr is masculine, and after w in this meaning, we need the locative:

• nominative: ten semestr
• locative: w tym semestrze

So both words change:

• ten → tym
• semestr → semestrze

Because after w meaning in/during, the noun goes into the locative, and the locative form of semestr is semestrze.

For this noun:

• nominative: semestr
• genitive: semestru
• locative: semestrze

So:

• w semestrze = correct
• w semestru = not correct here

Because mało usually requires the genitive.

So:

• czas = nominative
• czasu = genitive singular

That is why Polish says:

• mało czasu = little time
• not mało czas

This is very common in Polish with words like:

• dużo
• mało
• trochę
• ile
• wiele

Examples:

• dużo pracy = a lot of work
• mało pieniędzy = little money
• trochę wody = a little water

It can mean both, depending on context.

In this sentence, mamy mało czasu means:

• we have little time
• we don’t have much time

Both are natural English translations. The Polish phrase itself is neutral and common.

Because Polish often leaves out subject pronouns when the verb already shows who the subject is.

mamy already means we have, so my is not necessary.

• mamy mało czasu = we have little time
• my mamy mało czasu = also possible, but my adds emphasis, like we (as opposed to someone else)

Polish does this a lot:

• mam = I have
• masz = you have
• mamy = we have

Yes, both are possible, but they sound a little different.

• bo = because, very common in everyday speech
• ponieważ = because / since, a bit more formal or written

So:

• W tym semestrze mamy mało czasu, bo co tydzień jest kartkówka.
  natural, everyday
• W tym semestrze mamy mało czasu, ponieważ co tydzień jest kartkówka.
  also correct, slightly more formal

Co tydzień means every week.

This is a very common Polish pattern:

• co dzień / codziennie = every day
• co tydzień = every week
• co miesiąc = every month
• co rok = every year

You generally do not say każdy tydzień in this structure.
If you want to use każdy, you would normally say:

• w każdy tydzień is not natural here
• każdego tygodnia can work in some contexts, but co tydzień is the standard simple way to say every week

So for a learner, co tydzień is the phrase to remember.

In this time expression, co is followed by the accusative singular.

So:

• nominative: tydzień
• accusative: tydzień

For this noun, nominative and accusative look the same, which is why the form does not change visibly.

Similar examples:

• co miesiąc
• co rok
• co semestr

This pattern functions like an adverbial expression meaning every ...

Jest kartkówka means something like there is a quiz or a quiz takes place.

This structure focuses on the existence/occurrence of the event, not on who has it.

• co tydzień jest kartkówka = every week there is a quiz
• co tydzień mamy kartkówkę = every week we have a quiz

Both are correct, but the nuance is slightly different:

• jest kartkówka = the quiz exists/occurs each week
• mamy kartkówkę = we have to take a quiz each week

In this sentence, jest kartkówka sounds very natural.

Kartkówka is a short school quiz, usually written, often on a small amount of recent material.

It is not exactly the same as a big test or exam.

Roughly:

• kartkówka = short quiz / short class test
• sprawdzian = bigger test
• egzamin = exam

In many school contexts, kartkówka can also suggest something quick or even unannounced, depending on the teacher and situation.

Because the subject is singular: kartkówka.

So:

• jest kartkówka = singular
• są kartkówki = plural

In the sentence, the meaning is every week there is a quiz, so a singular noun and singular verb are used.

Yes. Polish word order is flexible, although some versions sound more natural than others depending on emphasis.

Original:

• W tym semestrze mamy mało czasu, bo co tydzień jest kartkówka.

Other possible versions:

• Mamy mało czasu w tym semestrze, bo co tydzień jest kartkówka.
• Bo co tydzień jest kartkówka, w tym semestrze mamy mało czasu.
  grammatically possible, but less neutral

The original sentence sounds natural because:

• it starts with the time frame: W tym semestrze
• then gives the main statement: mamy mało czasu
• then explains the reason: bo co tydzień jest kartkówka

Literally, it is in this semester, but in natural English you would usually just say this semester.

So:

• W tym semestrze mamy mało czasu
  literally: In this semester we have little time
• natural English: This semester we don’t have much time

Polish often uses a preposition where English may prefer a simpler time expression.

Yes. Co tydzień clearly means a repeated weekly event, so it can carry that sense of every single week, especially if the speaker is complaining.

The sentence has a mildly frustrated tone:

• We have little time this semester, because there’s a quiz every week.

So the weekly repetition is an important part of the meaning.