mado wo sukosi akete netara, kazatoosi ga yokute gussuri nemuremasita.

Because 窓 is the direct object of 開ける (to open).

• 窓を開ける = to open the window
• Here, を marks the thing being opened.

So 窓を少し開けて means opening the window a little / leaving the window slightly open.

少し means a little or slightly.

In this sentence, it modifies 開けて, so it means the speaker opened the window a little, not all the way.

• 窓を少し開けて = opened the window a little
• very natural English sense: left the window slightly open

The て-form is being used to connect actions.

Here, 窓を少し開けて寝た means:

• I opened the window a little and went to sleep
• or more naturally, I slept with the window slightly open

The て-form often links actions in sequence, but in sentences like this it can also imply the state created by the first action continues into the second action.

So this is not just open it, then later sleep. It suggests:

• the window was left slightly open
• then the speaker slept in that condition

〜たら is a conditional form that often means if, when, or after doing something, depending on context.

Here:

• 窓を少し開けて寝たら literally means something like
  when I slept after leaving the window slightly open
  or
  when I tried sleeping with the window slightly open

The second part gives the result:

• 風通しがよくてぐっすり眠れました
  the ventilation was good, and I slept soundly

So in this sentence, 〜たら introduces the situation that led to the result. A natural interpretation is:

• When I slept with the window slightly open, ...
• After leaving the window slightly open and going to sleep, ...

Not in this sentence.

Because 寝たら is past-form-based (寝た + ら) and the rest of the sentence talks about an actual result that happened, it is better understood as:

• when I slept
• after I slept
• when I tried sleeping

rather than a hypothetical if I sleep.

Japanese 〜たら can express a real past result very naturally:

• Xたら、Y happened

So here it is about a real experience, not a hypothetical situation.

風通し means airflow, ventilation, or how well air passes through a space.

So:

• 風通しがいい / よい = the ventilation is good
• 風通しが悪い = the ventilation is bad

In this sentence:

• 風通しがよくて = the airflow was good / because the ventilation was good

It refers to the room feeling fresher because the window was slightly open.

Because 風通し is the subject of よい (good), not a direct object.

• 風通しがよい = the ventilation is good
• が marks what has that quality

Then よくて is the て-form of よい / いい, used to connect to what follows.

So:

• 風通しがよくて、ぐっすり眠れました
• the ventilation was good, so I was able to sleep soundly

Yes. よくて is the て-form of よい (and いい is the common everyday form of よい).

The adjective changes like this:

• dictionary form: よい / いい
• adverb form: よく
• て-form: よくて

Here, よくて connects the idea the ventilation was good to the result I slept soundly.

It often carries a reason-like nuance:

• 風通しがよくて、ぐっすり眠れました
• The ventilation was good, so I slept soundly

ぐっすり is an adverb meaning:

• soundly
• deeply
• fast asleep

It is very commonly used with words for sleeping, such as:

• ぐっすり眠る = sleep soundly
• ぐっすり寝る = sleep deeply

So ぐっすり眠れました means I was able to sleep soundly or simply I slept soundly.

Yes, both can mean to sleep, but they have slightly different nuances.

• 寝る often focuses on going to bed, lying down, or sleeping
• 眠る focuses more on the actual state of being asleep

In this sentence:

• 開けて寝たら = when I went to sleep / slept with the window open
• ぐっすり眠れました = I was able to sleep soundly

Using both is very natural. It does not sound repetitive in Japanese because the nuance is a little different:

• first part: the action/situation of going to bed
• second part: the quality/result of the sleep itself

眠れました is the potential form of 眠る, so it literally means:

• was able to sleep

This often sounds very natural in Japanese when talking about sleeping well or badly. It can imply that circumstances allowed good sleep.

So:

• 眠りました = slept
• 眠れました = was able to sleep

In English, both may be translated naturally as:

• I slept soundly
• I was able to sleep soundly

But the Japanese version with 眠れました highlights that the good ventilation made good sleep possible.

Yes. 眠れました is in polite past form.

The plain version would be:

• 窓を少し開けて寝たら、風通しがよくてぐっすり眠れた。

Using 〜ました makes the sentence more polite and neutral, which is common in conversation, writing, and textbook examples.

It breaks down like this:

• 窓を少し開けて寝たら、
  when I slept with the window slightly open / after leaving the window slightly open and going to sleep,
• 風通しがよくて
  the ventilation was good, so
• ぐっすり眠れました。
  I was able to sleep soundly.

So the full flow is:

1. leave the window a little open
2. go to sleep in that condition
3. the airflow is good
4. as a result, sleep soundly

Yes, that is also a correct sentence.

• 窓を少し開けて寝ました。
  = I slept with the window slightly open.

But it does not include the later result/explanation about ventilation and sleeping well.

The original sentence with 〜たら adds a result discovered from doing that:

• When I slept with the window slightly open, the ventilation was good and I slept soundly.

So the original sounds more like a small personal discovery or observation.