W hotelu była miękka kołdra, czyste prześcieradło i wygodny materac.

Because the preposition w meaning in normally takes the locative case when it describes location.

• hotel = dictionary form
• w hotelu = in the hotel

So -u is the locative singular ending here.

Polish usually does not use a dummy subject like English there.

English says:

• There was a soft duvet...

Polish simply says:

• Była miękka kołdra...

So the idea of there was / there were is expressed by był / była / było / były plus the rest of the sentence. The word order helps give that existential meaning.

This is a very common question.

In a sentence like this, where the verb comes before a list of nouns, Polish often uses a singular verb that agrees with the first noun in the list.

Here the first noun is:

• kołdra — feminine singular

So the verb is:

• była — feminine singular

That is why you get:

• W hotelu była miękka kołdra, czyste prześcieradło i wygodny materac.

You may also hear or see były in similar sentences, because logically there are multiple things. But with this verb-first, presenting-a-list structure, singular agreement with the first noun is very natural.

If the list came first, plural agreement would be more expected:

• Miękka kołdra, czyste prześcieradło i wygodny materac były w hotelu.

In the Polish past tense, the verb agrees with gender and number.

Here are the relevant forms of być in the past:

• był — masculine singular
• była — feminine singular
• było — neuter singular
• były — non-masculine-personal plural
• byli — masculine-personal plural

So była tells you the verb is agreeing with something feminine singular — here, kołdra.

Because adjectives in Polish must agree with the nouns they describe in gender, number, and case.

Here:

• miękka kołdra — kołdra is feminine, so miękka
• czyste prześcieradło — prześcieradło is neuter, so czyste
• wygodny materac — materac is masculine, so wygodny

So the adjective endings change to match the noun.

They are:

• kołdra — feminine
• prześcieradło — neuter
• materac — masculine

That is exactly why the sentence has three different adjective forms:

• miękka kołdra
• czyste prześcieradło
• wygodny materac

Because they are not direct objects here. They are the things that were present / existed in the hotel.

After być in this kind of sentence, the nouns are normally in the nominative:

• była kołdra
• było prześcieradło
• był materac

Only w hotelu is in a different case, because the preposition w requires the locative for location.

Because Polish has no articles.

So Polish does not have separate words equivalent to English a/an and the. Whether something is definite or indefinite is understood from:

• context
• word order
• stress
• the situation

So kołdra can mean a duvet or the duvet, depending on context.

Because in a simple list, Polish normally does not put a comma before i (and), just like standard English usually does not.

So this is normal:

• miękka kołdra, czyste prześcieradło i wygodny materac

You use commas between earlier items in the list, but not before the final i in a basic coordination.

Not exactly.

• kołdra = usually duvet, quilt, or comforter, depending on the kind of English you use
• prześcieradło = bedsheet, especially the sheet you lie on

A regular blanket is more often:

• koc

So kołdra is bedding used for sleeping, but it is not the best direct match for a simple sofa blanket.

Yes. Polish word order is much more flexible than English, although different orders can sound more natural in different contexts.

This sentence starts with:

• W hotelu ...

That puts the location first, as the setting.

You could also say:

• Była w hotelu miękka kołdra, czyste prześcieradło i wygodny materac.
• Miękka kołdra, czyste prześcieradło i wygodny materac były w hotelu.

These are possible, but they do not feel exactly the same:

• W hotelu była... sounds like you are introducing what was in the hotel.
• ...były w hotelu sounds more like you are talking about those things and then saying where they were.

So the original word order is very natural for presenting the contents of the room or hotel.