natu ha kazatoosi no ii heya de hirune wo suru to, kimoti ga ii desu.

Here 夏は means something like as for summer or in summer.

The particle は marks 夏 as the topic. Japanese often uses a topic like this to set the scene:

• 夏は = as for summer / in summer
• 冬は = as for winter / in winter

So the sentence is not saying summer is... in a literal subject-predicate way. It is more like:

• When it’s summer, taking a nap... feels good
• In summer, it feels good to take a nap...

風通しのいい部屋 means a well-ventilated room or a room with good airflow.

It can be broken down like this:

• 風 = wind
• 通し = passing through
• 風通し = ventilation / airflow
• いい = good
• 部屋 = room

So literally, it is something like:

• a room of good airflow
• more naturally: a room with good ventilation

The whole phrase 風通しのいい modifies 部屋, just like well-ventilated modifies room in English.

Both ideas exist, but 風通しのいい部屋 is a very common and natural expression.

A useful way to think about it:

• 風通しがいい = the ventilation is good
• 風通しのいい部屋 = a room with good ventilation

So の helps turn 風通し into a modifier before いい, creating a set descriptive phrase.

You may also see 風通しがいい部屋, and it can be understood, but 風通しのいい部屋 is especially common as a noun-modifying phrase.

で marks the place where an action happens.

Here, the action is:

• 昼寝をする = to take a nap

So:

• 部屋で昼寝をする = to take a nap in a room

Japanese often uses:

• に for existence or destination
• で for the location of an action

Since taking a nap is treated as an action here, で is used.

Both are possible.

• 昼寝をする
• 昼寝する

Both mean to take a nap.

The version with をする can sound a little more explicit or slightly more formal/complete, because 昼寝 is a noun and をする turns it into a verb phrase.

This same pattern is very common:

• 勉強をする = to study
• 散歩をする = to take a walk
• 昼寝をする = to take a nap

In casual speech, 昼寝する is also very natural.

Here と means when or if, but specifically in a general-result sense.

So:

• 昼寝をすると、気持ちがいいです = when you take a nap, it feels good = if you take a nap, it feels good

This と is often used when the second part is a natural consequence, regular result, or general truth.

So this sentence has the feeling of:

• In summer, if you take a nap in a well-ventilated room, it feels good
• In summer, taking a nap in a breezy room feels nice

It is not mainly about one single specific event; it sounds more like a general observation.

気持ちがいい is a very common expression meaning:

• to feel good
• to feel pleasant
• to be comfortable
• to be refreshing

It does not necessarily mean emotional happiness. It often refers to a pleasant physical or sensory feeling.

Examples:

• A cool breeze feels nice.
• A hot bath feels good.
• Stretching feels good.
• Taking a nap in a breezy room feels pleasant.

So in this sentence, 気持ちがいいです means something like:

• it feels good
• it is pleasant
• it is comfortable

With 気持ちがいい, 気持ち is treated as the thing that is good/pleasant.

So the pattern is:

• X が いい = X is good

Here:

• 気持ちがいい = the feeling is good → it feels good

If you said 気持ちはいい, the nuance would change. は would create contrast, as if you were saying something like:

• As for the feeling, it’s good...
  maybe implying something else is not.

So が is the natural choice here.

It sounds like a general statement or a typical truth.

Several things give that feeling:

• 夏は sets up a seasonal topic
• すると often expresses a regular result
• the plain present forms are often used for habits or general facts

So the sentence feels like:

• In summer, taking a nap in a well-ventilated room feels good
• During summer, it’s nice to nap in a breezy room

If the speaker wanted to describe one specific past experience, the sentence would probably look different.

A helpful way to group it is:

• 夏は
• 風通しのいい部屋で
• 昼寝をすると、
• 気持ちがいいです。

Piece by piece:

• 夏は = as for summer / in summer
• 風通しのいい部屋で = in a well-ventilated room
• 昼寝をすると = when/if one takes a nap
• 気持ちがいいです = it feels good

So a natural full translation is:

• In summer, taking a nap in a well-ventilated room feels good.

Here です mainly makes the sentence polite.

It does not need to be translated as a separate is every time. The core meaning is already in:

• 気持ちがいい = feels good / is pleasant

Adding です makes it polite:

• 気持ちがいいです = it feels good (polite)

So yes, it is part of the polite predicate ending, but in practice its main job here is politeness rather than adding a new meaning.