Na drugim rondzie nie widziałam znaku i pojechałam nie tam, gdzie trzeba.

Polish normally says na rondzie for at/on the roundabout. Here na expresses location, and it takes the locative case, so:

• rondo → na rondzie
• drugie rondo → na drugim rondzie

So na drugim rondzie means at the second roundabout or on the second roundabout.

Using w drugim rondzie would sound wrong here, because w is not the normal preposition with rondo in this meaning.

Because it has to agree with rondzie in case, number, and gender.

The base form is:

• drugie rondo = the second roundabout

After na in a location meaning, rondo goes into the locative singular:

• na drugim rondzie

So:

• drugie → nominative/accusative neuter singular
• drugim → locative singular

This is just adjective agreement.

The ending -łam shows that the speaker is:

• female
• speaking about the past

So:

• widziałam = I saw / I was seeing said by a woman
• pojechałam = I went / drove off said by a woman

If the speaker were male, it would be:

• widziałem
• pojechałem

Polish past tense changes for gender in the singular.

Because after negation, Polish very often changes the direct object from the accusative to the genitive.

Compare:

• Widziałam znak. = I saw the sign.
• Nie widziałam znaku. = I didn’t see the sign.

So:

• affirmative: znak
• negative: znaku

This is a very important pattern in Polish, especially with verbs like widzieć.

Because they describe two different kinds of actions.

• widzieć is imperfective: it describes seeing as a state or ongoing perception.
• pojechać is perfective: it describes one completed action of going/driving somewhere.

So the sentence presents:

1. I didn’t see the sign
2. and I went the wrong way

That is very natural in Polish.

You could also hear nie zobaczyłam znaku, which would mean something closer to I failed to notice / I didn’t catch sight of the sign. But nie widziałam znaku is perfectly normal.

Literally, it is something like:

• not there where it is necessary
• not where one should

But in natural English it means:

• not where I was supposed to go
• the wrong way
• not to the right place/direction

The expression gdzie trzeba is idiomatic and very common. It means:

• where needed
• where appropriate
• where one should

So pojechałam nie tam, gdzie trzeba means I went not where I should have gone.

There is no normal subject. Trzeba is an impersonal word in Polish.

It means things like:

• it is necessary
• one must
• one should

So:

• Trzeba jechać prosto. = One should go straight.
• To nie jest miejsce, gdzie trzeba skręcić. = This is not the place where one should turn.

In gdzie trzeba, Polish leaves the subject unspecified. It has a general meaning: where one is supposed to.

Because gdzie trzeba is a subordinate clause.

Polish normally puts a comma before clauses introduced by words like:

• że
• który
• gdzie
• kiedy
• bo
• żeby

So here:

• pojechałam nie tam, gdzie trzeba

The comma separates:

• the main part: pojechałam nie tam
• the subordinate clause: gdzie trzeba

This comma is required in standard Polish spelling.

Because Polish usually drops subject pronouns when the verb already shows who the subject is.

Both verbs clearly show I:

• widziałam
• pojechałam

So ja is unnecessary unless you want emphasis or contrast.

For example:

• Ja nie widziałam znaku, tylko ty. = I didn’t see the sign, only you did.

Without emphasis, leaving out ja sounds more natural.

Because pojechałam focuses on a single completed movement: the speaker ended up going in the wrong direction/place.

• pojechałam = I went / drove off
• jechałam = I was driving / I drove with a more ongoing sense

Here the idea is not just that she was driving, but that she made the wrong move and went the wrong way, so pojechałam fits better.

Yes, that is possible, but it shifts the emphasis slightly.

• pojechałam nie tam, gdzie trzeba = I went not where I should have
• pojechałam tam, gdzie nie trzeba = I went where I shouldn’t have

Both can work, but the original version is especially natural when contrasting:

• the correct place/direction vs.
• the place/direction actually taken

So nie tam, gdzie trzeba is a very common and idiomatic way to express the wrong way/place.