Когда я открыл окно, в комнату залетела бабочка.

Because открыл is the perfective past tense of открыть. It presents the action as a completed event: I opened the window.

In this sentence, that completed action creates the moment when the next event happens:

• Когда я открыл окно... = When I opened the window...
• then: ...в комнату залетела бабочка = ...a butterfly flew into the room

If you used открывал, that would be imperfective, which usually suggests a process, repetition, or background action:

• Когда я открывал окно... = When I was opening the window...

That version is possible in some contexts, but it would shift the meaning slightly toward the action being in progress rather than completed.

Here когда means when and introduces a subordinate clause:

• Когда я открыл окно = When I opened the window

It connects the two events in time. In this sentence, it tells us that the butterfly flew in at the time that the speaker opened the window.

Because both verbs are perfective past tense, the sense is often close to:

• when
• once
• sometimes even as soon as, depending on context

But the basic translation is simply when.

Because Когда я открыл окно is a subordinate clause, and in Russian such clauses are normally separated from the main clause by a comma.

So the structure is:

• Когда я открыл окно, = subordinate clause
• в комнату залетела бабочка. = main clause

This is very standard Russian punctuation.

Because Russian uses different cases after в depending on whether you mean:

• movement into somewhere → Accusative
• location in somewhere → Prepositional

Here the butterfly is flying into the room, so Russian uses в + accusative:

• в комнату = into the room

Compare:

• в комнате = in the room, inside it, location
• в комнату = into the room, motion toward/into it

So:

• Бабочка залетела в комнату = The butterfly flew into the room
• Бабочка была в комнате = The butterfly was in the room

Залетела is the past tense feminine form of залететь.

Here it means:

• flew in
• flew into
• often with a nuance of coming in from outside, sometimes accidentally or unexpectedly

So в комнату залетела бабочка is not just that the butterfly was flying around in the room; it specifically means it entered the room by flying.

A useful comparison:

• летела = was flying / flew
• прилетела = arrived by flying
• залетела = flew in / flew into

The prefix за- here helps give the idea of entering.

Because the subject, бабочка, is feminine singular, and Russian past tense verbs agree with the subject in gender and number.

So:

• masculine: залетел
• feminine: залетела
• neuter: залетело
• plural: залетели

Since бабочка is feminine, the verb must be залетела.

This is one of the big differences from English: in the past tense, Russian verbs show gender in the singular.

Бабочка is in the nominative singular, because it is the subject of the verb залетела.

You can tell it is the thing doing the action:

• бабочка залетела = the butterfly flew in

Even though the word order puts бабочка at the end, it is still the subject.

Both are grammatical, but the word order changes the focus.

Neutral/basic order

• Бабочка залетела в комнату.
• A butterfly flew into the room.

Order used here

• В комнату залетела бабочка.
• Literally: Into the room flew a butterfly.

This version puts the emphasis first on where the butterfly went or on the event entering the room, and then introduces бабочка at the end. It can sound a bit more vivid or natural in storytelling.

Russian word order is more flexible than English because case endings help show grammatical roles.

It is accusative singular, because it is the direct object of открыл:

• открыть что? → окно

However, for this noun the accusative looks exactly like the nominative:

• nominative: окно
• accusative: окно

That is normal for many inanimate neuter nouns in Russian.

In this sentence, yes, it is natural to include я:

• Когда я открыл окно...

Russian often omits subject pronouns when they are obvious from context, but in a sentence like this, especially at the start of a story or statement, я is normally included.

Without it:

• Когда открыл окно...

that would usually sound incomplete by itself, unless the subject were already very clear from the surrounding context.

Because the whole sentence describes two events that happened in the past:

1. я открыл окно — I opened the window
2. бабочка залетела в комнату — a butterfly flew into the room

Russian uses past tense in both clauses just as English does here.

Also, both verbs are perfective, which helps show two completed events in sequence:

• first the window was opened
• then the butterfly flew in

That makes the sentence feel like a single finished episode.