Wieczorem obieram ziemniaki, a moja siostra używa tarki do sera.

Wieczorem means in the evening or sometimes this evening, depending on context.

There is no preposition because wieczorem is a fixed adverbial form used for time expressions. Polish often does this with parts of the day:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

So Wieczorem obieram ziemniaki is a natural way to say I peel potatoes in the evening.

Obieram is the 1st person singular present tense form of obierać.

So it means:

• I peel
• I am peeling

Polish present tense can cover both a general action and an action happening now, depending on context.

Polish usually leaves out subject pronouns when they are clear from the verb ending.

In obieram, the ending -am already tells you the subject is I. So ja is not necessary.

You could say Ja obieram ziemniaki if you want extra emphasis, for example:

• Ja obieram ziemniaki, a ona gotuje.
  I peel the potatoes, and she cooks.

Without emphasis, dropping ja is more natural.

Ziemniaki is the accusative plural here, because it is the direct object of obieram.

You are peeling what?
→ ziemniaki

For this noun, the accusative plural looks the same as the nominative plural:

• ziemniaki = potatoes

This is very common with inanimate masculine nouns in Polish.

It can mean either one.

Polish present tense often covers both:

• a habitual meaning: I peel potatoes
• a current action: I am peeling potatoes

The context decides which one is intended. In this sentence, either reading is possible unless the wider context makes it clearer.

In this sentence, a means something like:

• and
• while
• whereas

It often connects two related ideas with a slight contrast or comparison.

So:

• obieram ziemniaki, a moja siostra używa tarki...
  = I peel potatoes, and meanwhile / whereas my sister uses a grater...

If you used i, it would sound more like a simple addition of two facts. A is better here because the sentence compares what two different people are doing.

Moja siostra means my sister.

• moja = my
• siostra = sister

Moja changes its form to match siostra, which is feminine singular nominative.

So this is an example of adjective agreement: the possessive adjective must match the noun in gender, number, and case.

Because the verb używać usually takes the genitive case, not the accusative.

So:

• dictionary form: tarka = grater
• genitive singular: tarki

That is why you get:

• używa tarki = uses a grater

This is something learners often just need to memorize:

• używać + genitive

Literally, tarka do sera means a grater for cheese.

• tarka = grater
• do = for / for the purpose of
• sera = of cheese / cheese in the genitive

In natural English, you would usually say a cheese grater.

So Polish often expresses this idea as:

• noun + do + genitive
• tarka do sera = grater for cheese
• szklanka do wody = glass for water
• łyżka do zupy = spoon for soup

Because do requires the genitive case.

The basic form is:

• ser = cheese

After do, it changes to:

• sera

So:

• do sera = for cheese / of cheese

This is a normal case change after the preposition do.

Yes. Polish word order is fairly flexible.

You could also say:

• Obieram ziemniaki wieczorem, a moja siostra używa tarki do sera.

That still makes sense. But putting Wieczorem first gives the sentence a natural time frame at the beginning, like In the evening...

So the original word order is very natural and common.

It could be either, depending on context.

Polish present tense is often flexible in this way.

Possible readings:

• habit/routine: In the evening I peel potatoes, and my sister uses a cheese grater.
• current situation: This evening I’m peeling potatoes, and my sister is using a cheese grater.

If there were more context before or after the sentence, it would usually make the intended meaning clear.