Po spacerze ogon psa jest mokry od deszczu.

Because po in the sense of after requires the locative case in Polish.

• dictionary form: spacer = walk
• locative singular: spacerze

So:

• po spacerze = after the walk / after a walk

This is a very common pattern:

• po pracy = after work
• po obiedzie = after lunch
• po lekcji = after class

Spacerze is in the locative singular.

The base noun is spacer. After po meaning after, it changes to:

• spacer → spacerze

So in this sentence:

• Po spacerze = a prepositional phrase in the locative, meaning after the walk

Polish usually shows possession by putting the owner in the genitive case.

So:

• pies = dog
• psa = of the dog

That gives:

• ogon psa = the dog's tail / tail of the dog

English uses 's, but Polish normally uses a noun + genitive structure instead.

Here psa is genitive singular of pies.

Why genitive? Because ogon psa expresses possession:

• ogon = tail
• psa = of the dog

So the structure is literally:

• tail of the dog

Be careful: psa can also be accusative singular in other sentences, but here it is clearly genitive because it depends on ogon.

Because Polish has no articles.

English distinguishes:

• a dog
• the dog

Polish does not do that with separate words. Context tells you whether something is definite or indefinite.

So ogon psa can mean:

• the dog's tail
• a dog's tail

In this sentence, the meaning is naturally understood as the dog's tail.

Jest is the 3rd person singular form of być (to be), so it means is.

• ogon psa = the dog's tail
• jest = is
• mokry = wet

So:

• ogon psa jest mokry = the dog's tail is wet

In Polish, you normally use jest in this kind of sentence with an adjective.

Because the adjective must agree with the noun it describes.

The noun here is ogon (tail), which is:

• masculine
• singular

So the adjective also has to be masculine singular:

• mokry = wet

Compare:

• sukienka jest mokra = the dress is wet
• okno jest mokre = the window is wet

Because od here means something like from or because of, and it commonly appears in expressions describing the source of a condition.

So:

• mokry od deszczu = wet from the rain / wet because of the rain

Also, od requires the genitive case, which is why deszcz becomes deszczu.

Deszczu is the genitive singular of deszcz (rain).

That happens because od takes the genitive:

• deszcz = rain
• od deszczu = from the rain

So in the sentence:

• mokry od deszczu = wet from the rain

Usually, mokry od deszczu is the more natural choice here.

• od deszczu means from the rain / because of the rain
• z deszczu often suggests movement out of the rain or appears in other expressions

So for describing why something is wet, od deszczu is the standard and idiomatic phrasing.

That word order puts the time/context first: After the walk...

Polish word order is more flexible than English, because case endings show grammatical relationships. So several orders are possible, for example:

• Po spacerze ogon psa jest mokry od deszczu.
• Ogon psa jest mokry od deszczu po spacerze.

But the first version sounds more natural if you want to set the scene first.

No. A very natural alternative would be:

• Po spacerze pies ma mokry ogon od deszczu.

Literally:

• After the walk, the dog has a wet tail from the rain.

Both are correct. The original sentence focuses on the tail itself as the subject:

• ogon psa jest mokry...

The alternative focuses more on the dog:

• pies ma mokry ogon...

Yes. Since Polish has no articles, po spacerze can mean either:

• after a walk
• after the walk

Context decides which one is meant. In many real situations, either translation may sound natural in English.

Spacer most often means a walk, especially a casual walk for pleasure or fresh air.

So:

• iść na spacer = to go for a walk
• po spacerze = after the walk / after a walk

It is not usually used for something like a long hike in the mountains; for that, Polish often uses other words depending on the context.

It looks different because Polish nouns often change noticeably in different cases.

• base form: deszcz
• genitive: deszczu

That -u ending is a common genitive singular ending for some masculine nouns.

Pronunciation-wise, deszcz is already a tricky cluster, and deszczu adds another vowel at the end, making it easier to continue into the next part of the phrase:

• od deszczu

The spelling change is normal and is just part of case inflection in Polish.