Moja siostra woli tankować wieczorem, bo wtedy na stacji jest mniej ludzi.

Because moja siostra is the subject of the sentence, so it stays in the nominative case.

• moja = nominative feminine singular of mój
• siostra = nominative singular

You would use moją siostrę only when my sister is the direct object, for example:

• Widzę moją siostrę. = I see my sister.

Here, your sister is the one doing the action, so it must be moja siostra.

Woli is the 3rd person singular form of the verb woleć (to prefer).

So:

• ja wolę = I prefer
• ty wolisz = you prefer
• on/ona/ono woli = he/she/it prefers

Since the subject is moja siostra (my sister), Polish uses woli.

After woleć, Polish commonly uses an infinitive to say what someone prefers to do.

So the pattern is:

• woleć + infinitive

Examples:

• Wolę zostać w domu. = I prefer to stay at home.
• Ona woli czytać wieczorem. = She prefers to read in the evening.
• Moja siostra woli tankować wieczorem.

This works very much like English prefer to do.

This is about aspect.

• tankować = imperfective: the general activity, repeated or habitual action
• zatankować = perfective: one completed refueling

In this sentence, the speaker is talking about a general preference/habit, not one specific completed action, so tankować is the natural choice.

Compare:

• Moja siostra woli tankować wieczorem. = She generally prefers refueling in the evening.
• Moja siostra chce jutro zatankować. = She wants to refuel tomorrow (one completed act).

Wieczorem is the normal adverb-like form meaning in the evening / in the evenings.

It is historically the instrumental form of wieczór (evening), but learners often just memorize it as a fixed time expression.

Common time expressions like this include:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night
• latem = in summer
• zimą = in winter

So wieczorem is simply the standard way to say in the evening.

Yes, you could say just bo, but wtedy adds the sense of then / at that time.

• bo = because
• wtedy = then, at that time

So:

• bo na stacji jest mniej ludzi = because there are fewer people at the station
• bo wtedy na stacji jest mniej ludzi = because then / at that time there are fewer people at the station

It helps link the second clause specifically to the evening.

Because after na meaning at/on/in a place, Polish often uses the locative case.

The basic noun is:

• stacja = station

In the locative singular:

• na stacji = at the station

Here stacja means station, and in this context it usually means a gas/petrol station because of tankować.

Compare:

• Jestem na stacji. = I am at the station.
• Pracuje na stacji. = He/she works at the station.

There are two things going on here.

1. mniej normally requires the genitive After mniej (fewer / less), the following noun usually goes into the genitive:
   • ludzie = nominative plural
   • ludzi = genitive plural

So mniej ludzi is correct.

2. Polish often uses jest with quantity expressions With expressions like dużo, mało, mniej, więcej, Polish very often uses jest / było / będzie in the singular neuter-style construction, even though the meaning is plural.

So:

• Jest dużo ludzi.
• Jest mniej ludzi.
• Było wielu gości.

This is a very common pattern and is worth learning as a set.

Both can work, but mniej ludzi is very natural and common.

• ludzie / ludzi = people
• osoby / osób = persons / people

In everyday Polish:

• mniej ludzi sounds very normal in casual speech
• mniej osób can sound a little more neutral, formal, or count-focused

So this sentence could also be:

• ...bo wtedy na stacji jest mniej osób.

But mniej ludzi is probably the most natural everyday version.

Yes, it is irregular and very common.

The noun człowiek means person / human being, but the plural usually becomes:

• ludzie = people

Its genitive plural is:

• ludzi

So:

• ludzie są mili = people are nice
• nie ma ludzi = there are no people
• mniej ludzi = fewer people

This is one of those high-frequency forms that is best memorized early.

Yes. Polish word order is more flexible than English, because grammatical roles are shown by endings more than by position.

The original sentence is natural:

• Moja siostra woli tankować wieczorem, bo wtedy na stacji jest mniej ludzi.

But you could also hear:

• Wieczorem moja siostra woli tankować, bo wtedy na stacji jest mniej ludzi.
• Moja siostra woli wieczorem tankować, bo wtedy na stacji jest mniej ludzi.

These versions shift the focus slightly:

• putting wieczorem earlier emphasizes the time
• keeping it after the infinitive sounds neutral and natural

So the original version is a good standard one, but not the only possible order.

Usually, yes. Tankować means to fill up with fuel / to refuel.

In everyday use, it often implies:

• filling a car
• sometimes a motorcycle or other vehicle

Because of that, na stacji is naturally understood as at the gas/petrol station.

So even though stacja by itself can mean different kinds of station, the verb tankować makes the meaning clear.

Bo is very common, natural, and neutral in everyday Polish.

Other options include:

• ponieważ = because, since
• gdyż = because, for

So you could also say:

• Moja siostra woli tankować wieczorem, ponieważ wtedy na stacji jest mniej ludzi.

That sounds a bit more formal or written. In normal conversation, bo is usually the most natural choice.