Na każdym rondzie patrzę na znak i dopiero potem skręcam.

Because na can take different cases depending on meaning.

• na + locative is used for location: na rondzie = at/on the roundabout
• na + accusative is often used for movement toward something: na rondo = onto the roundabout

In your sentence, the speaker is talking about what they do while at each roundabout, so Polish uses the locative:

• na każdym rondzie

not the accusative.

Każdym is the form of każdy that matches rondzie.

Here, rondo is:

• neuter
• singular
• in the locative case

So the adjective/pronoun must match it:

• nominative: każde rondo
• locative: na każdym rondzie

Polish adjectives and words like każdy change form to agree with the noun in gender, number, and case.

Rondzie is the locative singular form of rondo.

You usually see the locative after certain prepositions, including:

• w
• na
• o
• przy
• po

So:

• rondo = roundabout
• na rondzie = at/on the roundabout

With każdym, you get:

• na każdym rondzie = at every roundabout

In Polish, the verb patrzeć normally goes with na + accusative when it means to look at something.

So:

• patrzeć na znak = to look at the sign

This is just the standard pattern for the verb. It is similar to English look at.

Compare:

• widzę znak = I see the sign
• patrzę na znak = I am looking at the sign

So patrzeć is about the action of directing your eyes, while widzieć is about perception.

Because patrzeć na takes the accusative case, and znak is an inanimate masculine noun.

For many inanimate masculine nouns in Polish, the accusative singular looks the same as the nominative singular.

So:

• nominative: znak
• accusative: znak

That is why you see:

• patrzę na znak

and not a different-looking form.

Yes, in this context it most naturally means a road sign / traffic sign.

The word znak can mean several things depending on context, such as:

• sign
• symbol
• mark
• indication

But with rondo and skręcam, the intended meaning is clearly traffic sign.

Dopiero potem means something like:

• only then
• only afterwards
• not until after that

It adds emphasis: the speaker first looks at the sign, and only after doing that do they turn.

So the sequence is:

1. patrzę na znak
2. dopiero potem skręcam

Without dopiero, potem would just mean then/afterwards.
With dopiero, the idea is stronger: I don’t turn before that.

Skręcam is the imperfective present form, and here it describes a habitual/repeated action:

• At every roundabout, I look at the sign and only then turn.

That is why present tense is natural.

• skręcam = I turn / I am turning / I usually turn

By contrast, skręcę is perfective future:

• skręcę = I will turn

So if you said skręcę, it would sound more like a single future event, not a general routine.

Because Polish usually does not need subject pronouns when the verb ending already shows who the subject is.

Here:

• patrzę = I look
• skręcam = I turn

The endings already tell you the subject is I, so adding ja is usually unnecessary.

You could say Ja patrzę..., but that would add emphasis, for example:

• Ja patrzę na znak, a nie ty.
  I’m the one who looks at the sign, not you.

In a neutral sentence, leaving out ja is normal.

Yes. In both Polish and English, the present tense is often used for habitual actions.

So this sentence means something like:

• Whenever I’m at a roundabout, I look at the sign and only then turn.
• At every roundabout, I look at the sign and then turn.

This is not necessarily happening right now. It is a statement about the speaker’s normal behavior.

No, Polish word order is relatively flexible, but the chosen order sounds natural and clear.

The sentence:

• Na każdym rondzie patrzę na znak i dopiero potem skręcam.

has a very natural flow:

1. setting: Na każdym rondzie
2. first action: patrzę na znak
3. second action with emphasis: i dopiero potem skręcam

You could rearrange it, for example:

• Patrzę na znak na każdym rondzie i dopiero potem skręcam.

That is still understandable, but it changes the emphasis slightly.
Starting with Na każdym rondzie highlights the situation right away.

Yes, i here simply means and.

It joins the two actions:

• patrzę na znak
• dopiero potem skręcam

So:

• patrzę na znak i dopiero potem skręcam
• I look at the sign and only then do I turn

The important nuance comes mostly from dopiero potem, not from i itself.

Sometimes yes, but potem is the most natural choice here.

• potem = then / afterwards
• później = later

You might hear:

• i dopiero potem skręcam
• i dopiero później skręcam

Both are possible, but dopiero potem sounds very natural for a simple sequence of actions.
Później can sound a bit more like later on, sometimes with slightly more distance in time.

The dictionary forms are:

• patrzeć = to look
• skręcać = to turn

Your sentence uses the 1st person singular present forms:

• patrzę = I look
• skręcam = I turn

So if you are learning vocabulary, the forms to remember are:

• patrzeć na
  • accusative = to look at
• skręcać = to turn

It can mean either in different contexts, but here it most naturally means I turn in traffic, for example taking an exit or changing direction at the roundabout.

The exact direction is not specified. The sentence only says that the speaker checks the sign first and then turns.

So in context, it means something like:

• I make the turn
• I take the correct exit / direction

Yes. Polish na often corresponds to English on or at, but you should not translate it too literally.

With roads, intersections, public spaces, and similar places, Polish often uses na where English might prefer at or something else.

So:

• na rondzie is the normal Polish way to say at the roundabout / on the roundabout

Even if on the roundabout sounds a bit strange in English, the Polish phrase is perfectly normal.

Yes, it sounds natural and idiomatic.

It clearly expresses a routine:

• At every roundabout, I look at the sign and only then turn.

The grammar, word order, and choice of expressions are all normal in everyday Polish.