Когда я открыл дверь, моя собака убежала из дома.

Because открыл is the perfective past tense of открыть, and it presents the action as a completed event: I opened the door.

In this sentence, the idea is that one completed action happened, and then another happened as a result:

• Когда я открыл дверь = When I opened the door
• моя собака убежала из дома = my dog ran out of the house

If you used открывал, it would sound more like was opening / used to open / opened repeatedly, depending on context. That does not fit as well here, because the sentence refers to one specific completed moment.

Убежала is also perfective. It means the dog ran away / got out / escaped as a completed action.

Compare:

• бежала = was running or ran in an ongoing sense
• убежала = ran away, with the result that the dog is now gone

The prefix у- often gives the idea of away. So убежала is not just movement; it is movement away from the place.

Because собака is grammatically feminine, and Russian past tense agrees with gender in the singular.

So:

• я открыл = I opened said by a male speaker
• я открыла = I opened said by a female speaker

And with собака:

• собака убежала = the dog ran away
  even if the dog is biologically male, the noun собака is usually treated as feminine grammatically

That is why the verb is убежала, not убежал.

Because собака is a feminine noun, and the possessive pronoun must agree with it.

So:

• мой = masculine
• моя = feminine
• моё = neuter
• мои = plural

Since собака is feminine, you say моя собака = my dog.

Because the preposition из requires the genitive case.

The noun дом changes in the genitive singular:

• nominative: дом
• genitive: дома

So:

• из дома = out of the house / from the house

This is a very common pattern:

• из школы = from school
• из магазина = from the store
• из комнаты = from the room

Here из дома means out of the house.

Literally:

• из = out of / from
• дома = of the house in the genitive case

So the dog did not just run somewhere; it ran out from inside the house.

English often uses out of the house, while Russian commonly uses just из дома.

Because Когда я открыл дверь is a subordinate clause introduced by когда = when.

Russian normally separates this kind of clause with a comma:

• Когда я открыл дверь, моя собака убежала из дома.

This is similar to English:

• When I opened the door, my dog ran out of the house.

If the order is reversed, Russian still uses a comma:

• Моя собака убежала из дома, когда я открыл дверь.

Yes. Russian word order is relatively flexible, though different orders can sound more neutral, more emphatic, or more literary.

The given sentence is very natural and neutral:

• Когда я открыл дверь, моя собака убежала из дома.

You could also say:

• Моя собака убежала из дома, когда я открыл дверь.

That still means basically the same thing, but the focus shifts slightly toward my dog ran out of the house first, with the when clause added after it.

Russian often uses word order to manage emphasis rather than basic grammar.

Russian has no articles like English a and the.

So:

• дверь can mean a door or the door
• собака can mean a dog or the dog
• дом can mean a house or the house

The exact meaning depends on context. In this sentence, English naturally uses the door and my dog, but Russian does not need separate words for that.

Sometimes yes, but here я is normally kept.

Russian can omit subject pronouns when the subject is obvious from context, especially in conversation. However, in the past tense, the verb form by itself does not always clearly show the person:

• открыл can mean I opened or he opened, depending on context

So including я makes the sentence clear:

• Когда я открыл дверь... = When I opened the door...

Without я, the sentence might sound incomplete or ambiguous unless the context already makes it obvious.

Дверь is in the accusative singular, but for this noun the accusative looks the same as the nominative.

The verb открыть takes a direct object:

• открыть что? = open what?
• дверь = door

So:

• nominative: дверь
• accusative: дверь

Because дверь is an inanimate feminine noun ending in a soft sign, these two forms are identical.

The dictionary forms are:

• открыть = to open
• убежать = to run away

The forms in the sentence are past tense singular:

• открыл = opened
• убежала = ran away

It is helpful to learn them together with aspect pairs:

• открывать / открыть = to open
• убегать / убежать = to run away

That helps you understand why this sentence uses the perfective forms.

In the past tense, Russian verbs show gender in the singular.

So:

• я открыл = I opened if the speaker is male
• я открыла = I opened if the speaker is female

The English translation does not show this, but Russian does.

So the sentence as written suggests that the speaker is male. If the speaker were female, it would be:

• Когда я открыла дверь, моя собака убежала из дома.

Not quite.

• вышла из дома = went out of the house / came out of the house
• убежала из дома = ran away out of the house

Убежала suggests more speed and often a sense that the dog got loose or escaped. It sounds more dramatic and fits the idea that the dog seized the chance and dashed out.

So убежала is stronger and more specific than вышла.

The stress is:

• когда́
• я
• откры́л
• две́рь
• моя́
• соба́ка
• убежа́ла
• из
• до́ма

A rough pronunciation guide:

• ког-DA
• at-kRYL
• DVYER'
• ma-YA
• sa-BA-ka
• oo-be-ZHA-la
• DO-ma

Stress matters in Russian, so it is worth memorizing words together with their stress.