tā wàngle dài yàoshi, zhǐhǎo zài ménkǒu děng shìyǒu lái kāimén.

Here, 了 marks that the forgetting happened as a completed event in this situation.

• 忘带钥匙 can sound more general, like forget to bring keys
• 忘了带钥匙 clearly refers to this specific incident: she forgot

So 了 is attached to 忘, not to 带.

Because she did not actually bring the key.

• 带钥匙 is the action she was supposed to do
• that action never happened
• so you normally do not add 了 to 带

If you said 带了钥匙, it would suggest that she did bring the key, which would contradict the meaning.

Yes. 忘 and 忘记 both mean to forget.

• 她忘了带钥匙
• 她忘记带钥匙了

Both are natural. In general:

• 忘 is a bit shorter and very common in speech
• 忘记 can sound slightly more formal or complete

In everyday Mandarin, both are used a lot.

带 here means to bring along / carry with you.

That makes it the natural choice when talking about leaving home without something you were supposed to have with you.

• 带钥匙 = bring the key with you
• 拿钥匙 = take/pick up the key

拿 is possible in some contexts, but 带 fits this situation better because the idea is that she went out and did not have the key with her.

It can mean either. Chinese nouns usually do not change form for singular and plural.

So 钥匙 could mean:

• a key
• keys

The exact meaning depends on context. In this sentence, English might translate it as the key or her key, but Chinese does not need to mark that explicitly.

只好 means to have no choice but to.

It shows that the speaker sees the action as the only practical option because of the situation.

So:

• 只好在门口等室友来开门 = she had no choice but to wait at the door for her roommate to come open it

This is stronger than just saying 就. It carries the feeling of unfortunately, this is the only thing she can do now.

Because Chinese often leaves out a repeated subject when it is already clear.

So after 她忘了带钥匙, the subject of 只好在门口等室友来开门 is still understood to be 她.

You could say:

• 她忘了带钥匙，她只好在门口等室友来开门。

But leaving out the second 她 sounds smoother and more natural.

In Chinese, location phrases often come before the main verb.

So:

• 在门口等 = wait at the door / wait by the entrance

This is the normal word order:

• 在 + place + verb

Also, 门口 literally means doorway / entrance area, not just the physical door itself.

This structure means:

• 等 + person + 来 + verb

So 等室友来开门 means:

• wait for the roommate to come and open the door

Breaking it down:

• 等室友 = wait for the roommate
• 来开门 = come open the door

The 来 shows movement toward where she is.

来 is used because the roommate is coming to the place where she is waiting.

From her point of view, the roommate needs to come to the door.

So:

• 室友来开门 = the roommate comes to open the door

If the action were moving away from the speaker’s location, Chinese might use 去 instead.

Because 开门 is the natural everyday expression for open the door.

In this situation, it can include the idea of unlocking it if necessary. Chinese often uses the simpler, more general phrase unless the lock itself is the focus.

• 开门 = open the door
• 开锁 = unlock the lock

Here, the important point is that she cannot get in, so she needs her roommate to open the door.

It is basically a problem → result sentence.

• 她忘了带钥匙 = the problem
• 只好在门口等室友来开门 = the result / what she has to do because of that problem

Chinese often links these ideas directly, without needing words like because or so. The relationship is clear from context and from the use of 只好.