Я не могу войти в подъезд, потому что домофон не работает.

Войти is the perfective infinitive of входить.

After мочь (can / be able to), Russian often uses:

• perfective if you mean manage to do a single complete action
• imperfective if you mean be able to do it in general, or if the focus is on the process

Here, не могу войти means I can’t get in / I can’t enter — the speaker is talking about one specific attempted action.

Compare:

• Я не могу войти в подъезд. = I can’t get into the entrance.
• Я не могу входить в этот подъезд. = I’m not allowed / not able to enter this entrance in general.

So войти is the natural choice here.

Because Russian uses different cases after в depending on meaning:

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

So:

• в подъезд = into the entrance
• в подъезде = in the entrance

Since the speaker is trying to enter, motion is involved, so подъезд is in the accusative case. For this noun, the accusative looks the same as the nominative: подъезд.

In this sentence, подъезд usually means the entrance area of an apartment building, often including the door from the street and the shared stairwell/lobby behind it.

It does not usually mean driveway here, even though English learners sometimes guess that from the look of the word.

A good practical translation is:

• building entrance
• entranceway
• sometimes simply entrance

In everyday Russian, especially in cities, подъезд is a very common word for the shared entrance to a residential building.

Домофон is the intercom / door-entry system at the entrance of a building.

It is the device that lets residents open the main door remotely or lets visitors call an apartment.

So домофон не работает means the intercom system is not functioning, which is why the speaker can’t get inside.

Russian very often uses не работает (doesn’t work / isn’t working) for machines, devices, and systems.

So:

• Телефон не работает. = The phone isn’t working.
• Лифт не работает. = The elevator isn’t working.
• Домофон не работает. = The intercom isn’t working.

You could also hear:

• Домофон сломан. = The intercom is broken.
• Домофон сломался. = The intercom broke / has broken.

But не работает is especially common when the speaker is focused on the practical result: it does not function.

Because потому что introduces a subordinate clause:

• main clause: Я не могу войти в подъезд
• reason clause: потому что домофон не работает

In standard Russian spelling, a comma is normally placed before потому что, just as English often separates a main statement from a following reason clause.

This is a very common question.

• потому что = because
• поэтому = therefore / so / that’s why

Your sentence uses потому что because it gives the reason:

• Я не могу войти в подъезд, потому что домофон не работает. = I can’t get into the entrance because the intercom isn’t working.

If you used поэтому, the structure would change:

• Домофон не работает, поэтому я не могу войти в подъезд. = The intercom isn’t working, so I can’t get into the entrance.

So the meaning is similar, but the grammar and sentence structure are different.

Могу is the first-person singular form of мочь (to be able / can).

Conjugation of мочь is somewhat irregular:

• я могу = I can
• ты можешь = you can
• он / она может = he / she can
• мы можем = we can
• вы можете = you can
• они могут = they can

Because the subject is Я, you need могу.

Yes. Russian often drops subject pronouns when the verb already makes the subject clear.

So both are natural:

• Я не могу войти в подъезд...
• Не могу войти в подъезд...

The version with Я may sound a bit more explicit or emphatic, while the version without it is often more conversational.

The hard sign ъ is not pronounced as a separate sound, but it affects pronunciation.

In подъезд, it shows that:

• the prefix под- stays separate
• the following е is pronounced like ye
• the consonant before it is not softened

So подъезд is pronounced roughly like pad-YEZD.

Without the hard sign, the pronunciation pattern would be different. The ъ is often used after prefixes before е, ё, ю, я.

Because не normally goes directly before the verb it negates.

Here it negates могу:

• могу = I can
• не могу = I cannot / I can’t

So the sentence literally starts with I not can enter...

This is the normal Russian way to form verbal negation.

Russian word order is more flexible than English, although the version in your sentence is the most neutral and natural.

Neutral order:

• Я не могу войти в подъезд, потому что домофон не работает.

You can change the order for emphasis, for example:

• В подъезд я не могу войти, потому что домофон не работает.
  • emphasis on into the entrance
• Я не могу войти, потому что домофон не работает.
  • if в подъезд is already understood

So the sentence order can change, but the original version is the safest default for learners.

Yes, the meaning is different.

• не могу войти = I can’t enter; something prevents me
• не вхожу = I am not entering / I don’t enter

So не могу войти clearly expresses inability, not just the fact that the action is not happening.

In this sentence, the reason is given: the intercom doesn’t work. That makes не могу войти exactly right.

Войти comes from the verb family related to идти (to go on foot).

Very roughly:

• идти = to go
• входить = to go in / enter, imperfective
• войти = to go in / enter, perfective

The prefix в- adds the idea of into.

This is a very common pattern in Russian verbs of motion:

• идти = go
• войти = go in
• выйти = go out
• зайти = drop in / go in
• прийти = come / arrive

So войти literally has the sense of go in, but in English it is often best translated simply as enter or get in.