Stąd widać rzekę, a stamtąd widać już tylko most i parking.

They are both adverbs of place:

• stąd = from here
• stamtąd = from there

They point to a place you are talking about as a source or viewing point. In this sentence, they tell you where the view is from.

So:

• Stąd widać rzekę = from here, the river is visible
• Stamtąd widać... = from there, you can see...

They are single words, not combinations like z + tutaj/tam.

Widać is a very common impersonal form in Polish. It means something like:

• you can see
• one can see
• it is visible

It does not name a specific person who is doing the seeing. That is why it works well here: the sentence is about what is visible from a certain place, not about what I or you personally see.

Compare:

• Stąd widzę rzekę = I can see the river from here
• Stąd widać rzekę = The river can be seen from here / You can see the river from here

So widać makes the statement more general and natural.

Because the things being seen are in the accusative case.

After widać, the visible object is typically in the accusative:

• rzeka → rzekę
• most → most
• parking → parking

The reason only rzeka visibly changes is that it is a feminine singular noun. Its accusative form is different from the nominative.

But most and parking are masculine inanimate singular nouns. For that group, the accusative is usually the same as the nominative, so there is no visible change.

Tylko means only.

Już often means already, but in sentences like this it adds the idea of by that point, now, or any longer except for.

So już tylko most i parking suggests that from that other place, the view is more limited: you no longer see the river; at that point you see only the bridge and the parking lot.

A natural English sense is:

• from there, you can only see the bridge and the parking lot
• or from there, all you can see is the bridge and the parking lot

A often links two clauses with a mild contrast, comparison, or change of viewpoint.

Here the sentence compares two locations:

• from here you can see the river
• from there you can only see the bridge and parking lot

That slight contrast makes a the natural choice.

If you used i, it would sound more like simple addition: this and that. A is better when English would often use and, but with a subtle sense of whereas or while.

Because a joins two separate clauses:

• Stąd widać rzekę
• stamtąd widać już tylko most i parking

In Polish, when a connects two finite clauses, a comma is normally used.

So the comma here is standard punctuation.

The word order is fairly flexible, but this version is very natural.

Starting with stąd and stamtąd puts emphasis on the two viewing points:

• Stąd widać rzekę...
• stamtąd widać już tylko...

You could change the order, for example:

• Rzekę stąd widać
• Most i parking widać stamtąd

But that changes the emphasis and may sound less neutral in this context. The original sentence is a clear, natural way to contrast what is visible from two places.

No. They are adverbs, not prepositions.

That means they do not govern a noun case the way a preposition would. They simply mean from here and from there by themselves.

So in this sentence:

• stąd does not control rzekę
• stamtąd does not control most i parking

The case of rzekę / most / parking comes from the construction with widać, not from stąd or stamtąd.

Yes. In Polish, parking is a normal noun meaning parking lot or car park.

So most i parking means the bridge and the parking lot/car park.

It does not mean the activity of parking in this sentence.