W mieście powietrze czasem nie pachnie tak dobrze jak w lesie.

Because Polish uses the locative case after the preposition w when it means “in” (static location).

• miasto (city) → locative: w mieście
• las (forest) → locative: w lesie

So:

• w mieście = in the city
• w lesie = in the forest

Using w miasto / w las would be wrong for location; those forms would only appear in very specific, different structures (e.g., with motion verbs and other prepositions).

Pachnie is the 3rd person singular, present tense form of the verb pachnieć (to smell in the sense of to give off a smell).

• Infinitive: pachnieć – to smell (give off a scent)
• On / ono pachnie – it smells

Important nuance:

• pachnieć usually implies a neutral or pleasant smell.
• For an unpleasant smell, Poles very often use śmierdzieć (to stink), e.g.
  
  • Powietrze śmierdzi – The air stinks.
  • or Powietrze brzydko pachnie – The air smells bad.

Also:

• wąchać = to smell in the sense of to sniff something with your nose (an active action).

Because here we’re describing how the air smells (an action of the verb), not what it is like as a noun.

• dobrze is an adverb – used with verbs:
  
  • pachnie dobrze – it smells good / well
• dobry / dobre are adjectives – used with nouns:
  
  • dobre powietrze – good air

In the sentence:

• powietrze nie pachnie tak dobrze
  = the air does not smell as good

We are modifying pachnie (the verb), so we need the adverb dobrze, not the adjective dobre.

tak … jak is a common Polish way to make equality comparisons, like “as … as” in English.

• tak dobrze jak = as well / as good as

In the whole phrase:

• nie pachnie tak dobrze jak w lesie
  = doesn’t smell as good as (it does) in the forest

Other examples:

• On biega tak szybko jak ona. – He runs as fast as she does.
• To nie jest tak trudne jak myślisz. – It’s not as hard as you think.

Polish has no articles (no equivalent of “a/an/the”). Whether English would use “air” or “the air” is understood from context.

• powietrze can mean both air and the air.
• Here, W mieście powietrze… is naturally understood as “the air in the city”.

If you need to be very specific, you use other words, not articles:

• to powietrze – this air
• tamto powietrze – that air
• miejskie powietrze – city air, urban air

Yes, that is correct and natural:

• Powietrze w mieście czasem nie pachnie tak dobrze jak w lesie.

This version puts a bit more emphasis on powietrze (the air) as the topic:
“The air in the city sometimes doesn’t smell as good as in the forest.”

The original:

• W mieście powietrze czasem…
  sounds slightly more like: “In the city, the air sometimes…”, highlighting the location at the start.

Polish word order is relatively flexible; changing it usually changes focus/emphasis, not the basic meaning.

Yes, czasem (sometimes) is mobile, and several positions are natural:

• W mieście powietrze czasem nie pachnie tak dobrze jak w lesie.
• W mieście czasem powietrze nie pachnie tak dobrze jak w lesie.
• Powietrze w mieście czasem nie pachnie tak dobrze jak w lesie.

All are fine and mean the same thing.

What you wouldn’t normally say:

• Powietrze nie pachnie czasem tak dobrze jak w lesie. – sounds odd; it might temporarily suggest “does it sometimes smell as good…?” and then get corrected by context.

Also, czasem and czasami are near-synonyms:

• czasem = sometimes
• czasami = sometimes (slightly more “full”/formal, but both are very common)

Both w mieście and w lesie use the locative singular.

Base (nominative) forms:

• miasto (n.) – city
  
  • locative: (w) mieście
• las (m.) – forest
  
  • locative: (w) lesie

You typically use:

• w
  • locative for static location:
    
    • w mieście – in the city
    • w lesie – in the forest
    • w domu – at home
    • w szkole – at school

Approximate pronunciation (using English-like hints):

• w mieście ≈ [v MYESH-tye]

  • w = like English “v” (often very light before m)
  • mie ≈ “mye”
  • ś is a soft “sh” (like “sh” but with the tongue closer to śi sound)
  • cie ≈ “chye”

• w lesie ≈ [v LYESH-ye]

  • l = clear “l” (not dark “ł”)
  • e = like “e” in “bet”
  • sie ≈ “shye” (soft “ś” + “e”)

The letters ś, ć, ź, ń are soft consonants, and combinations like sie, cie, zie, nie sound very similar to śe, će, źe, ñe.

Yes, that’s correct:

• Powietrze w mieście pachnie gorzej niż w lesie.
  = The air in the city smells worse than in the forest.

Differences:

• nie pachnie tak dobrze jak
  
  • literally “doesn’t smell as good as”
  • focuses on lack of equal goodness.
• pachnie gorzej niż
  
  • literally “smells worse than”
  • focuses directly on being worse.

Both are natural; it’s mostly a stylistic choice, similar to English:

• “doesn’t smell as good as” vs “smells worse than.”