Samochód stoi w garażu, a jego bagażnik jest otwarty.

Because samochód is the subject of the first clause, so it is in the nominative case.

• base form: samochód
• gender: masculine inanimate
• number: singular

In Polish, the subject of a sentence is normally in the nominative, just like car in the car is standing in English.

Stoi comes from stać, which means to stand.

With vehicles, Polish often uses stać where English would usually say is parked or simply is:

• Samochód stoi w garażu = the car is standing/parked in the garage

You can say Samochód jest w garażu, but that just means the car is in the garage.
Stoi sounds more natural when talking about a car being stationary somewhere.

Because after w when it means in for a location, Polish usually uses the locative case.

So:

• dictionary form: garaż
• locative singular: garażu

That is why you get w garażu = in the garage.

It is the locative singular form of garaż.

The locative is used after certain prepositions, especially when talking about location, such as:

• w domu = in the house
• na stole = on the table
• w garażu = in the garage

In this sentence, a means something like:

• and
• while
• whereas
• sometimes even a mild but

It connects two clauses and often suggests a slight contrast or a shift of focus.

So here it is not just a plain linker. It feels a bit like:

• The car is in the garage, and meanwhile its trunk is open.

Both can often translate as and, but they are not always interchangeable.

• i = simple addition
• a = addition with a slight contrast, change of topic, or comparison

In this sentence, a sounds natural because the speaker first says one thing about the car, then adds another related fact about it.
Using i would not be impossible, but a is better stylistically here.

Because in Polish, when a joins two clauses, it is normally preceded by a comma.

So:

• Samochód stoi w garażu, a jego bagażnik jest otwarty.

This is standard Polish punctuation.

Here jego means its.

It refers back to samochód, so:

• jego bagażnik = its trunk

The same word jego can also mean his, depending on context.
Since the owner here is a car, the natural English translation is its.

Because jego is one of the Polish possessive forms that usually stay the same in shape.

So you get:

• jego samochód
• jego bagażnik
• jego drzwi

Even though the noun after it may change case, gender, or number, jego itself usually does not change form the way many adjectives do.

Bagażnik means trunk in the car sense, or boot in British English.

It comes from bagaż, meaning luggage or baggage.

So:

• jego bagażnik = its trunk / its boot

Because otwarty is an adjective meaning open, and it must agree with bagażnik.

Since bagażnik is:

• masculine
• singular
• nominative

the adjective must also be:

• otwarty

Compare:

• bagażnik jest otwarty = the trunk is open
• drzwi są otwarte = the doors are open
• brama jest otwarta = the gate is open

Because jest is the normal present-tense form of to be in a full neutral sentence.

So:

• bagażnik jest otwarty = the trunk is open

In some styles, especially notes, headlines, or very informal speech, Polish may omit jest, but in a normal complete sentence it is standard to keep it.

Yes. Polish word order is more flexible than English word order.

For example, you could also say:

• W garażu stoi samochód, a jego bagażnik jest otwarty.

That version puts more focus on where the car is.

You could also move words around for emphasis, but the original sentence is very natural and neutral.

No. Polish does not have articles like English a and the.

So:

• samochód can mean a car or the car
• garaż can mean a garage or the garage
• bagażnik can mean a trunk or the trunk

The exact meaning comes from context.

A rough pronunciation guide is:

• Samochód ≈ sa-MO-hoot
• stoi ≈ STO-ee
• w garażu ≈ v ga-RA-zhoo
• jego ≈ YE-go
• bagażnik ≈ ba-GAZH-neek
• jest otwarty ≈ yest ot-VAR-tih

A few useful sound notes:

• w sounds like English v
• ż sounds like the s in measure
• ó sounds like u
• stress in Polish is usually on the second-to-last syllable

So the sentence is roughly:

sa-MO-hoot STO-ee v ga-RA-zhoo, a YE-go ba-GAZH-neek yest ot-VAR-tih