Jeśli benzyna będzie tańsza, zatankujemy samochód przed podróżą.

Yes. Jeśli and jeżeli both mean if.

• jeśli is very common and slightly more everyday
• jeżeli can sound a bit more formal or careful

So these are both natural:

• Jeśli benzyna będzie tańsza, zatankujemy samochód przed podróżą.
• Jeżeli benzyna będzie tańsza, zatankujemy samochód przed podróżą.

The meaning is the same.

This is a very common difference between English and Polish.

In Polish, when you talk about a future condition, it is normal to use the future tense in the if-clause:

• Jeśli benzyna będzie tańsza... = If petrol is cheaper / if petrol becomes cheaper...

Literally, it looks like If petrol will be cheaper..., but that is just how Polish expresses future conditions.

So:

• English: If it rains, we’ll stay home.
• Polish: Jeśli będzie padać, zostaniemy w domu.

This is one of those places where Polish grammar does not match English grammar directly.

Because tańsza has to agree with benzyna.

Benzyna is:

• singular
• feminine

So the adjective must also be:

• singular
• feminine

That gives:

• tańsza = cheaper, feminine singular

Compare:

• tańszy samochód = a cheaper car
• tańsza benzyna = cheaper petrol
• tańsze paliwo = cheaper fuel

This is basic adjective agreement in Polish.

Benzyna specifically means petrol / gasoline.

So for a British learner:

• benzyna = petrol

For an American learner:

• benzyna = gas / gasoline

It does not usually mean fuel in general. For fuel more generally, Polish often uses:

• paliwo = fuel

So:

• benzyna jest tańsza = petrol is cheaper
• paliwo jest tańsze = fuel is cheaper

Zatankujemy is the 1st person plural future form of zatankować.

It means:

• we will fill up
• we will refuel
• we’ll tank up

The verb zatankować is perfective, so it refers to a completed action: one whole act of filling the tank.

That is why zatankujemy sounds right here: it means that if the condition is met, we will do that one complete action before the trip.

Compare:

• tankować = to be refuelling / to refuel in general / repeated action
• zatankować = to fill up, to refuel completely once

So zatankujemy samochód means we’ll fill up the car.

In Polish, subject pronouns are often omitted because the verb ending already tells you who is doing the action.

Here:

• zatankujemy already means we will fill up

The ending -my shows we.

So:

• zatankujemy samochód = we will fill up the car

You can add my for emphasis, contrast, or clarity:

• My zatankujemy samochód, a oni pojadą pociągiem. = We’ll fill up the car, and they’ll go by train.

But in a normal sentence, leaving out my is completely natural.

Samochód is the direct object of zatankujemy, so it is in the accusative case.

The dictionary form is:

• samochód = car

For a masculine inanimate noun like samochód, the accusative singular is the same as the nominative singular:

• nominative: samochód
• accusative: samochód

So even though the form looks unchanged, it is still functioning as the accusative here.

That is why:

• Zatankujemy samochód. = We’ll fill up the car.

Yes, it is normal and idiomatic in Polish.

Polish commonly says:

• zatankować samochód
• zatankować auto

This means to fill up the car / refuel the car.

English often focuses on the tank:

• fill up the car
• fill the tank
• refuel the car

Polish can simply use the car as the object:

• zatankować samochód

So although it may sound slightly unusual if translated word-for-word, it is perfectly standard Polish.

Because the preposition przed here requires the instrumental case.

The noun is:

• podróż = trip, journey

Its instrumental singular is:

• podróżą

So:

• przed podróżą = before the trip

This is a common pattern:

• przed obiadem = before lunch
• przed pracą = before work
• przed spotkaniem = before the meeting
• przed podróżą = before the trip

So the form podróżą is there because of case, not because the meaning changes.

Because Jeśli benzyna będzie tańsza is a subordinate clause introduced by jeśli.

In Polish, such clauses are normally separated by a comma:

• Jeśli benzyna będzie tańsza, zatankujemy samochód przed podróżą.

Even if the order changes, the comma stays:

• Zatankujemy samochód przed podróżą, jeśli benzyna będzie tańsza.

This is standard Polish punctuation.

Yes. Polish word order is more flexible than English, although not completely free.

The neutral order here is:

• Jeśli benzyna będzie tańsza, zatankujemy samochód przed podróżą.

But you could also say:

• Zatankujemy samochód przed podróżą, jeśli benzyna będzie tańsza.

Both are natural.

Changing word order can shift emphasis a little:

• starting with Jeśli... foregrounds the condition
• starting with Zatankujemy... foregrounds the action

So the sentence can move around more than in English, while the core meaning stays the same.

Yes, but the meaning becomes broader.

• benzyna = petrol / gasoline specifically
• paliwo = fuel in general

So:

• Jeśli benzyna będzie tańsza... means the price of petrol/gasoline is lower

• Jeśli paliwo będzie tańsze... means fuel is cheaper more generally

Notice the adjective changes too:

• benzyna is feminine, so tańsza
• paliwo is neuter, so tańsze

So this would be correct:

• Jeśli paliwo będzie tańsze, zatankujemy samochód przed podróżą.

Yes, and it would sound natural, but there is a small nuance.

• przed podróżą = before the trip / before the journey
• przed wyjazdem = before departure / before leaving

So:

• przed podróżą focuses on the trip as a whole
• przed wyjazdem focuses more on the moment of setting off

In many everyday situations, both would work well. The original sentence with przed podróżą is completely natural.