Learn Hardcore Japanese: natu ha kazatoosi no ii heya de hirune wo suru to, kimoti ga ii desu. - In summer, taking a nap in a well-ventilated room feels good.

Breakdown of natsu wa kazatooshi no ii heya de hirune o suru to, kimochi ga ii desu.

What does 夏は mean here? Is it summer is... or in summer...?

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:

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

How does 風通しのいい部屋 work? What does it literally mean?

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:

So literally, it is something like:

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

Why is it 風通しのいい and not just 風通しがいい?