W zimie jemy mandarynki, bo mają dużo witaminy C.

Because after w in the meaning in (for location or time), Polish normally uses the locative case. The noun zima changes like this:

• nominative: zima
• locative: zimie

So w zimie means in winter.

Yes. Zimą is also a very common way to say in winter.

So both are correct:

• W zimie jemy mandarynki.
• Zimą jemy mandarynki.

For many learners, zimą sounds a bit more natural and compact in everyday Polish, but w zimie is absolutely correct.

Because Polish often leaves out subject pronouns when they are already clear from the verb ending.

• jemy = we eat
• my jemy = we eat, but with extra emphasis on we

So jemy by itself already tells you the subject is we. Adding my is only needed for contrast or emphasis.

Because mandarynki is the accusative plural form used as the direct object of jemy (we eat).

The verb jeść takes a direct object in the accusative:

• jem mandarynkę = I eat a mandarin
• jemy mandarynki = we eat mandarins

For this noun, the nominative plural and accusative plural happen to look the same:

• nominative plural: mandarynki
• accusative plural: mandarynki

So even though the form looks unchanged, grammatically it is functioning as the object.

Bo means because. It is very common in everyday spoken and written Polish.

• bo = common, neutral, everyday
• ponieważ = also because, but often a bit more formal
• gdyż = more formal/bookish

So in this sentence, bo is the most natural everyday choice.

Because in Polish, a clause introduced by bo is normally separated by a comma.

So:

• W zimie jemy mandarynki, bo mają dużo witaminy C.

This is standard Polish punctuation.

The subject is understood from context: it refers to mandarynki.

So the second clause really means:

• (mandarynki) mają dużo witaminy C
• (they / mandarins) have a lot of vitamin C

Polish often omits subjects when they are obvious. The verb ending -ą tells you it is 3rd person plural: they have.

Because dużo (a lot of) normally requires the genitive case.

So:

• dużo + genitive

The base form is witamina C, but after dużo it becomes:

• nominative: witamina C
• genitive: witaminy C

That is why the sentence says dużo witaminy C.

Because here Polish is talking about a lot of vitamin C as a substance/nutrient, not many vitamins in general.

So:

• dużo witaminy C = a lot of vitamin C
• dużo witamin = a lot of vitamins

The first one is singular because vitamin C is treated as one substance. The second is plural because it refers to multiple vitamins.

No, not here. In the expression witamina C, the noun changes, but the letter name C stays the same in normal writing:

• witamina C
• witaminy C
• witaminie C

So only witamina declines.

Polish word order is flexible, but the original sentence is a very natural neutral order:

• W zimie jemy mandarynki, bo mają dużo witaminy C.

You could also say:

• Mandarynki jemy w zimie, bo mają dużo witaminy C.

That version puts more focus on mandarynki. It is still correct, but the original sounds more neutral.

Because names of seasons in Polish are normally written with a lowercase letter:

• zima = winter
• wiosna = spring
• lato = summer
• jesień = autumn

So w zimie is correct, not w Zimie.

In everyday Polish, mandarynka / mandarynki usually refers to the small easy-to-peel citrus fruit people often call mandarin or sometimes tangerine in English. Exact fruit categories do not always match perfectly between Polish and English everyday usage.

So for learning Polish grammar, the important point here is the noun form:

• singular: mandarynka
• plural: mandarynki