Иногда собака лает просто потому, что слышит шаги в подъезде.

Because собака is the subject of the sentence: the dog is the one doing the action.

• собака = nominative singular
• It answers who barks? → собака

So in Иногда собака лает..., собака is simply the dog as the subject.

Лает is the correct 3rd person singular present-tense form of лаять (to bark).

Conjugation:

• я лаю
• ты лаешь
• он / она лает
• мы лаем
• вы лаете
• они лают

Since собака is singular and means she/it barks, the form is лает.

Russian often uses the present tense to talk about general, repeated, or habitual actions, just like English does.

So:

• Иногда собака лает... = Sometimes the dog barks...

This does not mean the dog is barking right now. It means this happens from time to time.

The same is true for слышит:

• что слышит шаги = because it hears footsteps
• In context, this means when it hears footsteps, as a repeated situation.

Иногда means sometimes. It is an adverb of frequency.

In this sentence it comes first:

• Иногда собака лает...

That is very natural, but Russian word order is flexible. You could also say:

• Собака иногда лает...

Both are correct. Putting иногда first gives it a little more emphasis as the setting for the whole sentence.

Просто потому, что means something like:

• simply because
• just because

It introduces the reason.

So:

• собака лает просто потому, что слышит шаги
  = the dog barks simply because it hears footsteps

Here просто adds the idea that the reason is plain or uncomplicated: there is no deeper cause.

This is a very common question.

Normally, потому что can function as a single conjunction meaning because. But when words like просто, лишь, именно, and similar words emphasize потому, Russian often splits the conjunction:

• просто потому, что
• лишь потому, что
• именно потому, что

So the comma appears before что.

In this sentence:

• лает просто потому, что слышит шаги

That punctuation is standard and natural.

Шаги is in the accusative plural, because it is the direct object of слышит (hears).

• слышать что? → шаги

For inanimate plural nouns, the accusative form is usually the same as the nominative form.

So:

• nominative plural: шаги
• accusative plural: шаги

That is why the form does not visibly change here.

Шаги literally means steps or footsteps. Russian commonly uses the plural here, just as English often says footsteps.

• слышит шаги = hears footsteps

Using the singular шаг would usually mean a step as one single step, which would sound different.

It is the prepositional case.

The preposition в can take different cases depending on meaning:

• в + accusative = motion into somewhere
• в + prepositional = location inside somewhere

Here the meaning is location:

• в подъезде = in the entryway / in the apartment building entrance / in the stairwell

So:

• подъезд → dictionary form
• в подъезде → prepositional singular

Подъезд is a very common Russian word, especially in the context of apartment buildings.

It usually means the building entrance area or stairwell section of an apartment block. Depending on context, English translations can include:

• entryway
• entrance hall
• building entrance
• stairwell

So шаги в подъезде means the dog hears someone moving around in that shared entrance/stairwell area.

Russian has no articles, so there is no direct equivalent of a or the.

That means:

• собака can mean a dog or the dog
• шаги can mean footsteps or the footsteps, depending on context

English has to choose an article, but Russian usually leaves that to context.

Yes. Russian word order is flexible, and the basic meaning would stay the same, though the emphasis may shift.

For example:

• Иногда собака лает просто потому, что слышит шаги в подъезде.
• Собака иногда лает просто потому, что слышит шаги в подъезде.
• Собака лает иногда просто потому, что слышит шаги в подъезде.

The first version is the most neutral and natural here.

Russian often uses word order to manage emphasis rather than basic grammar, since case endings already show many relationships.

Слышит is imperfective and fits the idea of a general, repeated situation.

The sentence means:

• sometimes the dog barks because it hears footsteps

This is a habitual pattern, not one single completed event. That is why imperfective слышит works best.

A perfective form like услышит would suggest a single completed act of hearing:

• when it hears / catches sound of something once

That would change the feel of the sentence.