Если я не закрою дверь, собака может убежать.

Закрою is future tense: I will close.

It comes from the verb pair:

• закрывать = to close (imperfective)
• закрыть = to close / to shut completely (perfective)

Russian perfective verbs do not have a true present tense. Their “present-looking” forms actually refer to the future. So:

• я закрываю = I am closing / I close
• я закрою = I will close

In this sentence, Russian uses закрою because it means a single completed action: if I don’t close the door.

This is a very common difference between English and Russian.

In Russian, after если (if), you can use a future form when the sentence is about a future situation:

• Если я не закрою дверь... = If I don’t close the door...

Even though English uses a present form (don’t close), Russian naturally uses the future here because the action refers to what will happen later.

So this is normal Russian, not a special exception.

Both are possible in some contexts, but they mean slightly different things.

• не закрою = I won’t close / I don’t close it (as a completed action)
• не буду закрывать = I won’t be closing / I’m not going to close it (more process-oriented or less focused on completion)

In this sentence, the important thing is whether the door ends up closed or not, so the perfective verb закрыть is the natural choice:

• Если я не закрою дверь, собака может убежать.

That means: if the action of closing is not completed, the dog may run away.

Дверь is the direct object of закрою: you close the door.

The noun дверь is feminine and inanimate. In the singular, its accusative form is the same as its nominative form:

• nominative: дверь
• accusative: дверь

So after закрою you still see дверь, not a different-looking form.

Russian has no articles like a or the.

So:

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

Context tells you which one is meant. In this sentence, the meaning is naturally understood as the door and the dog, but Russian does not need separate words for that.

After может (can / may / might), Russian normally uses an infinitive:

• может убежать = can run away / may run away

So the structure is:

• может = can/may
• убежать = to run away

You do not use a finite future verb like убежит after может.

Compare:

• Собака убежит. = The dog will run away.
• Собака может убежать. = The dog may/can run away.

This is another aspect pair:

• убегать = to run away, to be running away, to run away repeatedly (imperfective)
• убежать = to run away successfully / as a completed event (perfective)

Here, убежать is used because the idea is a single possible result: the dog might get away.

So:

• собака может убежать = the dog may run away / escape

If you used убегать, the focus would be less on the completed result and more on the process or repeated behavior.

It can correspond to several English words depending on context.

• может literally comes from мочь = to be able
• in context, it can mean can, may, or might

In this sentence, the most natural English idea is something like:

• the dog may run away
• the dog might run away
• sometimes also the dog can run away

It expresses possibility, not certainty.

Russian uses a comma to separate the if-clause from the main clause:

• Если я не закрою дверь, собака может убежать.

This is standard punctuation in Russian with subordinate clauses introduced by если.

English also usually uses a comma when the if-clause comes first:

• If I don’t close the door, the dog may run away.

Yes. Russian word order is more flexible than English word order, because case endings and verb forms help show the grammatical relationships.

The neutral order here is:

• Если я не закрою дверь, собака может убежать.

But other orders are possible for emphasis, for example:

• Если дверь не закрою я, собака может убежать.
  This emphasizes I.
• Собака может убежать, если я не закрою дверь.
  This puts the main result first.

Even though the order can change, the original version sounds very natural and neutral.

Собака is the subject of the second clause, so it is in the nominative case.

Basic form:

• собака = dog

Because it is the one that may run away, it stays in nominative:

• собака может убежать

The main stresses are:

• Е́сли
• я
• не закро́ю дверь
• соба́ка мо́жет убежа́ть

Word by word:

• Е́сли
• закро́ю
• дверь (one syllable)
• соба́ка
• мо́жет
• убежа́ть

Stress is important in Russian, because it can affect pronunciation quite a lot.

Usually, not in this context.

• Если я не закрою дверь... = If I don’t close the door...
  This refers to one future completed action.
• Если я не закрываю дверь... sounds more like
  If I am not closing the door or If I don’t usually close the door

So for a specific future situation, закрою is the correct and natural choice.

It is the main word for if in conditional sentences.

Examples:

• Если будет дождь, мы останемся дома. = If it rains, we’ll stay home.
• Если я не закрою дверь, собака может убежать.

So yes, если is the standard word you should learn first for if.

It expresses a condition plus a possible result.

• Если я не закрою дверь = if I don’t close the door
• собака может убежать = the dog may run away

So the speaker is not saying the dog definitely will run away. They are saying it is a real possibility if the door remains open.