Kiedy jest mgła, idę wolniej, żeby nie wejść w błoto przy chodniku.

Here kiedy introduces a time clause. In this sentence, it can be understood as when or whenever in English.

Because the sentence describes a repeated situation, whenever there is fog is often the clearest translation idea. But when there is fog is also perfectly fine.

So:

• Kiedy jest mgła = When/Whenever there is fog

Polish often does not use a separate word like English dummy there in expressions such as there is or there are.

So:

• jest mgła literally looks like is fog
• but it means there is fog

This is a normal Polish structure. The noun mgła is in the nominative case here.

This is a very common question because iść and chodzić can both relate to going on foot.

The basic difference is:

• iść = to go/walk in a specific direction, in a concrete situation
• chodzić = to walk/go habitually, repeatedly, or in a more general sense

In this sentence, idę wolniej gives the feeling of a concrete situation: when it is foggy, I am walking more slowly.

If you said chodzę wolniej, it would sound more like a general habit: I walk more slowly as a repeated behavior. That is not impossible, but idę fits the sentence’s situational feel better.

Wolniej is the comparative form of wolno and means more slowly or slower.

So:

• idę wolno = I walk slowly
• idę wolniej = I walk more slowly / slower

The sentence suggests a comparison with normal walking speed: because of the fog, I go slower than usual.

You could also say idę powoli for I walk slowly, but wolniej specifically emphasizes slower than usual.

Żeby introduces a purpose clause. Here it means so that, so as to, or in order to.

So:

• żeby nie wejść w błoto = so as not to step into the mud / in order not to step into the mud

It explains the reason for walking more slowly.

A more formal alternative is aby, but żeby is very common and natural in everyday Polish.

This is about aspect.

• wejść = perfective
• wchodzić = imperfective

In this sentence, the speaker wants to avoid a single completed event: accidentally stepping into the mud even once. That is why wejść is natural.

So:

• żeby nie wejść w błoto = so as not to step into the mud

If you used nie wchodzić, it would sound more like avoiding the action in a general or repeated sense, or like a general instruction: not to be going into the mud.

Because w can take different cases depending on meaning:

• w + accusative = movement into something
• w + locative = being inside something

Here the idea is movement: stepping into the mud.

So:

• wejść w błoto = step into the mud
• być w błocie = be in the mud

The noun błoto is neuter, and its nominative and accusative forms are the same, so you see błoto in both forms. But grammatically, here it is accusative.

Literally, wejść means to enter/go into, but in everyday Polish it is also used for things like stepping into mud, a puddle, or something unpleasant by accident.

So wejść w błoto is a very natural way to say:

• step into the mud
• walk into the mud

It does not necessarily mean dramatically enter it with your whole body. In context, it usually means putting your foot in it.

The preposition przy takes the locative case.

The dictionary form is:

• chodnik = sidewalk / pavement

After przy, it becomes:

• przy chodniku = by the sidewalk / next to the sidewalk

So chodniku is the locative singular form of chodnik.

Przy chodniku means by the sidewalk, next to the sidewalk, or along the sidewalk depending on context.

In this sentence, it suggests that the mud is located near the edge of the sidewalk, not necessarily on the sidewalk itself.

So the image is something like:

• there is mud beside the sidewalk
• I walk more slowly so I do not step into it

The commas separate subordinate clauses, which is standard in Polish.

The sentence has:

• a time clause: Kiedy jest mgła
• a main clause: idę wolniej
• a purpose clause: żeby nie wejść w błoto przy chodniku

So the commas help show the structure:

• Kiedy jest mgła, ...
• ..., żeby nie wejść w błoto przy chodniku.

In Polish, clauses introduced by words like kiedy and żeby are normally separated by commas.

Yes. Polish word order is more flexible than English word order, because case endings carry a lot of grammatical information.

This sentence is very natural as written:

• Kiedy jest mgła, idę wolniej, żeby nie wejść w błoto przy chodniku.

But other orders are possible, for example:

• Idę wolniej, kiedy jest mgła, żeby nie wejść w błoto przy chodniku.

The original order sounds natural because it presents:

1. the situation first: when there is fog
2. the action next: I walk more slowly
3. the purpose last: so as not to step into the mud