Я не смог войти в квартиру, потому что забыл ключ.

Не смог means was not able / couldn’t manage to, and it usually points to a specific completed situation.

• Я не смог войти в квартиру = I couldn’t get into the apartment in this particular instance.
• Я не мог входить в квартиру or Я не мог войти can also exist, but they mean something slightly different:
  • не мог often focuses more on inability in general or in context
  • не смог emphasizes failed to succeed

Here, the sentence describes one completed event: I tried to enter, but failed. So не смог is the most natural choice.

Because войти is the perfective form, and the sentence is about successfully entering once.

• войти = to enter / to get in as a completed action
• входить = to be entering / to enter regularly / to enter in general

In this sentence, the speaker means:

• I could not get in (complete the action)

So войти is correct.

Compare:

• Я не смог войти в квартиру. = I couldn’t get into the apartment.
• Я не мог входить в квартиру. = I was not allowed / able to enter the apartment (more general or repeated)

Because after в, Russian uses different cases depending on meaning:

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

So:

• в квартиру = into the apartment
• в квартире = in the apartment

Since the sentence is about trying to go into the apartment, Russian uses the accusative case:

• квартира → квартиру

Because ключ is the direct object of забыл.

• забыть что? = to forget what?
• Answer: ключ

For an inanimate masculine noun like ключ, the accusative singular looks the same as the nominative singular:

• nominative: ключ
• accusative: ключ

So забыл ключ simply means forgot the key.

Because забыл is perfective, and it refers to a single completed fact: the speaker realized that he had forgotten the key.

• забыть = to forget (perfective)
• забывать = to forget / be forgetting (imperfective)

In this sentence, the forgetting is treated as one finished event that caused the problem.

• Я забыл ключ. = I forgot the key.
• Я забывал ключ. is generally not natural here; it would suggest repeated or habitual forgetting in some contexts.

So забыл is the normal choice.

Russian has no articles like a or the.

So Russian simply says:

• ключ = key / the key / a key
• квартира = apartment / the apartment / an apartment

The exact meaning is understood from context. In this sentence, English naturally says the apartment and the key, but Russian does not need separate words for that.

Yes, very often.

Russian verb forms usually show the person clearly, so this sentence could also be:

• Не смог войти в квартиру, потому что забыл ключ.

This still clearly means I couldn’t get into the apartment because I forgot the key.

However, including Я can sound more explicit, more contrastive, or simply clearer depending on context.

Потому что is the standard, very common way to say because.

So:

• Я не смог войти в квартиру, потому что забыл ключ.
• = I couldn’t get into the apartment because I forgot the key.

Other options exist, but they are used differently:

• так как = since / because (often a bit more formal or bookish)
• из-за того что = because of the fact that (heavier style)

For normal everyday speech, потому что is the most natural choice.

Very close, yes, but Russian often makes the idea of failure to achieve a result more explicit.

• не смог войти literally means did not manage to enter
• In natural English, that is often simply couldn’t get in or couldn’t enter

So the meaning is basically the same, but Russian is very naturally using perfective verbs to show that the action did not reach completion.

Both can often mean to go into the apartment, but there is a nuance.

• войти = to enter
• зайти = to go in / drop in / stop by / come in

Войти is more neutral and directly about crossing into the space.

Зайти often suggests:

• going in briefly
• stopping by
• entering as part of a visit

Here, since the point is simply that the speaker could not enter, войти is the best choice.

Yes. Russian word order is flexible, although the original sentence is very natural.

Original:

• Я не смог войти в квартиру, потому что забыл ключ.

Other possible orders include:

• Я забыл ключ, поэтому не смог войти в квартиру. = I forgot the key, therefore I couldn’t get into the apartment.
• Потому что я забыл ключ, я не смог войти в квартиру. = Because I forgot the key, I couldn’t get into the apartment.

The original version is the most neutral and common conversational order.

Russian often leaves out possessive words like my, your, his, etc. when the meaning is obvious.

So:

• забыл ключ usually means forgot my/the key depending on context

If needed, Russian can say:

• забыл свой ключ = forgot my own key
• забыл мой ключ is much less natural in most situations

In this sentence, it is obvious that the speaker means the key needed to enter, so Russian simply says ключ.

A few stress points may help:

• Я
• не смог
• войти́
• в кварти́ру
• потому́ что
• забы́л ключ

A useful pronunciation note:

• что in потому что is often pronounced more like што in normal speech.

So the phrase often sounds approximately like:

• патаму́ што

That is normal spoken Russian pronunciation.