Я заглянул в комнату и увидел, что окно открыто.

Because в комнату expresses movement/direction: into the room.

Russian often uses:

• в + accusative = motion into something
• в + prepositional = location in something

So:

• Я заглянул в комнату = I looked/peeked into the room
• Я был в комнате = I was in the room

Here the idea is not that the speaker was located in the room, but that they directed their glance into it.

Заглянул comes from заглянуть, which often means:

• to glance into
• to peek into
• to look in
• sometimes to drop by briefly, depending on context

In this sentence, Я заглянул в комнату means something like:

• I looked into the room
• I peeked into the room

It suggests a quick look, not a long, careful examination.

Because заглянул is perfective, showing a single completed action.

• заглянул = took a look / looked in once
• заглядывал = was looking in / used to look in / looked in repeatedly

In this sentence, the speaker did one completed action and then immediately noticed something:

1. заглянул
2. увидел

That sequence strongly favors the perfective verbs.

Увидел is the perfective form and means caught sight of / saw as a completed event.

Compare:

• увидел = saw, noticed
• видел = saw, was seeing, had seen (more about the fact or experience of seeing)

In this sentence, the meaning is that after peeking into the room, the speaker noticed that the window was open. That moment of noticing is exactly what увидел expresses well.

There is no comma before и because заглянул and увидел are two verbs with the same subject Я. They function as coordinated predicates in one main clause:

• Я заглянул в комнату и увидел ...

But there is a comma before что because что окно открыто is a subordinate clause:

• увидел, что окно открыто
• literally: saw that the window was open

So:

• no comma before и in this structure
• comma before что because it introduces a dependent clause

Here что means that and introduces the content of what was seen/noticed:

• увидел, что окно открыто = saw/noticed that the window was open

This is very common in Russian after verbs like:

• знать = to know
• думать = to think
• сказать = to say
• увидеть = to see
• понять = to understand

So что links the main clause to the statement being reported.

Because открыто here is the short form of the adjective/passive-state form, used as a predicate: it describes the state of the window.

• окно открыто = the window is open / was open

By contrast:

• открытое окно = an open window

So:

• открытое = full form, usually used before a noun
• открыто = short form, often used after the noun as the predicate

This is a very common pattern in Russian:

• дверь закрыта = the door is closed
• магазин открыт = the store is open
• письмо написано = the letter is written

Because окно is neuter singular.

The short form agrees with the noun in gender and number:

• дом открыт — masculine
• дверь открыта — feminine
• окно открыто — neuter
• окна открыты — plural

Since окно is neuter, the correct form is открыто.

Russian normally omits the verb to be in the present tense:

• Окно открыто = The window is open

In English you must say is, but in Russian you usually do not.

In a past narrative like this one, окно открыто can still naturally mean the window was open, because the time reference is understood from the context: the speaker looked in and at that moment saw the window in an open state.

You could also sometimes hear что окно было открыто, but that is not necessary here.

In isolation, окно открыто most naturally means the window is open.

But inside this sentence, after past-tense verbs, English often translates it as the window was open, because that was the state at the time the speaker looked.

So the Russian form stays the same, while English chooses the tense that sounds natural in context.

Yes, Russian often omits subject pronouns when they are clear from the verb form.

So you could say:

• Заглянул в комнату и увидел, что окно открыто.

That would still mean I looked into the room and saw that the window was open, assuming the context makes the subject clear.

Including Я is also normal. It can make the sentence feel:

• a bit clearer
• more explicit
• more natural as a standalone sentence

So both are possible; Я is not wrong or unusual here.

Yes. Заглянул is masculine singular past tense, so it suggests the speaker is male.

If the speaker were female, it would be:

• Я заглянула в комнату и увидела, что окно открыто.

In the past tense, Russian verbs agree with gender in the singular:

• masculine: заглянул, увидел
• feminine: заглянула, увидела
• neuter: заглянуло (for neuter subjects)
• plural: заглянули, увидели

Yes. Я заглянул в комнату и увидел, что окно открыто is a very neutral, natural word order.

Russian word order is flexible, but the default order here is straightforward:

1. subject — Я
2. verb — заглянул
3. destination/object of glance — в комнату
4. second verb — увидел
5. subordinate clause — что окно открыто

You can change the order for emphasis, but this version is the most standard and easiest for learners.

Yes, very often.

In this sentence, English noticed may actually sound more natural:

• I looked into the room and noticed that the window was open.

That is because увидел does not always mean only physical seeing. It can also mean becoming aware of something by seeing it.

So depending on context, both are possible:

• saw that...
• noticed that...

They mean different things:

• окно открыто = the window is open
  → this describes a state
• окно открылось = the window opened
  → this describes an event/change

In your sentence, the speaker is describing what they saw: the window in an open state. So окно открыто is the correct choice.

It is definitely looked into the room.

English look in the room usually means search inside the room, which would be a different idea.

Russian заглянуть в комнату means directing your eyes into the room, often briefly from outside or from the doorway. So looked into the room or peeked into the room is the best match.