Wieczorem jeszcze raz sprawdzam bagażnik, bo często zostawiam tam zakupy.

Yes. Wieczorem comes from wieczór (evening), but here it functions as an adverb meaning in the evening / in the evenings.

Polish often uses time words in a special adverb-like form without a preposition:

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

So Wieczorem here means in the evening, not just the noun evening by itself.

Because Polish usually leaves out subject pronouns when the verb ending already shows who is doing the action.

Here:

• sprawdzam = I check
• zostawiam = I leave

The ending -am already tells you it is I. Adding ja is possible, but it usually adds emphasis or contrast:

• Ja sprawdzam bagażnik = I’m the one who checks the trunk

In a neutral sentence, leaving out ja is normal.

Jeszcze raz means one more time or again.

So:

• jeszcze raz sprawdzam bagażnik = I check the trunk one more time / again

A useful comparison:

• jeszcze raz = one more time, once again
• znowu = again
• ponownie = again, more formal

In this sentence, jeszcze raz sounds very natural because it suggests an extra repeated check.

Because the sentence describes a habitual/repeated action, not a single completed action.

• sprawdzam is imperfective present: I check / I am checking
• sprawdzę is perfective future: I will check

Here the meaning is something like:

• In the evening, I check the trunk again, because I often leave groceries there.

This is a routine, so Polish uses the imperfective sprawdzam.

It is in the accusative, because it is the direct object of sprawdzam.

You check what?

• bagażnik

For a masculine inanimate noun like bagażnik, the accusative singular looks the same as the nominative singular:

• nominative: bagażnik
• accusative: bagażnik

So the form does not change, even though the case is accusative.

Here bagażnik means the car trunk / boot.

It can mean a luggage compartment in general, but in this sentence that is clearly the intended meaning, especially because of:

• zostawiam tam zakupy = I leave groceries there

So the picture is: someone checks the car trunk because they often forget shopping in it.

Yes, you could use ponieważ, but bo is more natural in everyday speech.

• bo = because, common and conversational
• ponieważ = because, more formal
• gdyż = because, quite formal/bookish

So:

• ...bo często zostawiam tam zakupy sounds very natural
• ...ponieważ często zostawiam tam zakupy is correct, but less casual

Also, the comma before bo is normal and expected in Polish.

Tam means there or in there.

In this sentence, it refers back to bagażnik:

• zostawiam tam zakupy = I leave groceries there / in the trunk

It is not absolutely necessary, but it sounds natural because it avoids repeating w bagażniku.

Compare:

• zostawiam tam zakupy = I leave groceries there
• zostawiam zakupy w bagażniku = I leave groceries in the trunk

Both are correct.

Yes, zakupy is plural in form. In Polish, it often means:

• shopping
• the groceries
• the things you bought

Very often, English uses a singular idea where Polish uses a plural form.

Examples:

• robić zakupy = to do shopping
• zostawić zakupy w samochodzie = to leave the groceries in the car

Here zakupy means something like groceries / shopping bags / the things I bought.

It is in the accusative plural, because it is the direct object of zostawiam.

You leave what?

• zakupy

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

• nominative: zakupy
• accusative: zakupy

So again, the case is accusative, even though the form does not change.

Polish word order is fairly flexible, and this sentence could be rearranged. The basic meaning would stay the same, but the emphasis would change.

Original:

• Wieczorem jeszcze raz sprawdzam bagażnik, bo często zostawiam tam zakupy.

This puts emphasis first on when it happens: in the evening.

Possible alternatives:

• Jeszcze raz wieczorem sprawdzam bagażnik...
• Bagażnik sprawdzam jeszcze raz wieczorem...
• Wieczorem sprawdzam jeszcze raz bagażnik...

The original version sounds very natural and balanced.

In Polish, the present tense is exactly what you use for habits and repeated actions.

So:

• sprawdzam = I check / I do check / I am checking
• zostawiam = I leave / I tend to leave

The word często (often) makes it especially clear that zostawiam describes a repeated habit:

• bo często zostawiam tam zakupy = because I often leave groceries there

So the present tense here works just like English present simple for routines.