Na poddaszu latem bywa gorąco, dlatego wolę pokój na drugim piętrze.

Bywa comes from bywać, which often expresses something that happens from time to time, habitually, or under certain conditions.

So:

• Na poddaszu latem jest gorąco = It is hot in the attic in summer
• Na poddaszu latem bywa gorąco = It tends to get hot in the attic in summer / It can be hot in the attic in summer

Using bywa makes the statement less absolute and more like a general observation.

In Polish, words like gorąco, zimno, ciepło, ciemno are often used in impersonal expressions about conditions or weather:

• Jest gorąco = It is hot
• Bywa zimno = It can get cold

Here, gorąco is not describing a noun directly. It functions more like a predicative adverb, so gorące would not fit.

You would use gorące only if it agreed with a neuter noun, for example:

• to jest gorące danie = this is a hot dish

This is mostly a matter of Polish usage and idiom. With poddasze the normal expression for location is na poddaszu.

So Polish says:

• na poddaszu = in/on the attic space

Even though English uses in the attic, Polish uses na here.

Also, after na in a location meaning, Polish uses the locative case, which is why poddasze becomes poddaszu.

Poddaszu is in the locative singular.

Why?

• the preposition na can take different cases
• when it means location (on/in/at), it takes the locative
• so na poddaszu means in the attic / in the attic area

Base form:

• poddasze = nominative

After na for location:

• na poddaszu = locative

Latem means in summer.

Grammatically, it is an old case form used adverbially. For a learner, the important thing is that it behaves like a fixed time expression.

Similar words are:

• zimą = in winter
• wiosną = in spring
• jesienią = in autumn

So Polish often expresses seasons without a preposition:

• latem = in summer

not usually w lato in this meaning.

Because dlatego introduces the result or consequence of the first clause:

• Na poddaszu latem bywa gorąco = cause/reason
• dlatego wolę pokój... = result: that’s why I prefer...

So the comma separates two linked clauses, much like in English:

• It gets hot in the attic in summer, so/therefore I prefer...

Dlatego means therefore, for that reason, or that’s why.

In this sentence, it is very close to that’s why.

• dlatego emphasizes the reason-result link a bit more clearly
• więc often means so

Both can work in many contexts, but dlatego sounds slightly more explicit:

• ..., dlatego wolę... = ..., that’s why I prefer...
• ..., więc wolę... = ..., so I prefer...

Because woleć usually takes a direct object in the accusative case.

So:

• pokój is the dictionary form
• in the accusative singular, masculine inanimate nouns like pokój usually stay the same
• therefore: wolę pokój

Compare:

• mam pokój
• widzę pokój
• wolę pokój

All use the accusative.

Again, Polish uses na with floors of a building.

So:

• na parterze = on the ground floor
• na pierwszym piętrze = on the first floor
• na drugim piętrze = on the second floor

After na in a location meaning, the noun goes into the locative:

• piętro → piętrze

And the adjective must agree with it:

• drugie piętro → na drugim piętrze

Both are in the locative singular.

Why?

• na
  • location requires the locative
• piętro becomes piętrze
• the adjective drugi must agree with the noun in case, number, and gender
• because piętro is neuter singular, we get drugim

So:

• nominative: drugie piętro
• locative: na drugim piętrze

Be careful: the numbering may differ depending on the variety of English.

In Polish:

• parter = ground floor
• pierwsze piętro = the floor above the ground floor
• drugie piętro = the next one above that

So:

• in British English, drugie piętro usually matches the second floor
• in American English, it usually matches the third floor

This is a very common source of confusion.

Polish word order is flexible, but this version sounds natural because it moves from place to time to general condition:

• Na poddaszu = location
• latem = time
• bywa gorąco = what happens there

Other orders are possible, for example:

• Latem na poddaszu bywa gorąco

That also sounds natural. The difference is mainly in emphasis:

• Na poddaszu latem... emphasizes the place first
• Latem na poddaszu... emphasizes the time first