Which word is the subject noun?

[Grace Spreader]

In the sentence:

Several celebrities in the parking lot could be seen from our plane.

Isn't the subject noun an 'understood' missing word? Who is doing the seeing? The curriculum answer key shows that 'celebrities' is the subject noun. I should know better but can't understand how the curriculum is claiming this. Anyone know the rules behind this that you can elucidate for me?

[susiedqq]

Several celebrities in the parking lot could be seen from our plane.

basic bones of this sentence:

celebrities / could be seen

[Grace Spreader]

Several celebrities in the parking lot could be seen from our plane.

basic bones of this sentence:

celebrities / could be seen

My question was not how to separate this sentence but rather which word is the subject. The celebrities are not doing the action so, doesn't that make the word 'celebrities' an object as opposed to the subject? If 'celebrities' is the subject, please, help me understand why, since the action is done to them not by them.

[5jj]

Some passengers could see several celebrities.
Several celebreities could be seen.

In the first sentence, "(Some) passengers" is the grammatical subject of the verb - active voice.
In the second sentence "(Several) celebrities" is the grammatical subject of the verb - passive voice.

[Grace Spreader]

Some passengers could see several celebrities.
Several celebreities could be seen.

In the first sentence, "(Some) passengers" is the grammatical subject of the verb - active voice.
In the second sentence "(Several) celebrities" is the grammatical subject of the verb - passive voice.

Ah, yes, the passive voice. It has been too long since I learned about that. Thank you for your perfect explanation.

[philo2009]

Don't fall into the trap of assuming that a grammatical subject has to be performing some kind of action!

There are different terms (respectively, agent and patient) to denote those who act as opposed to those who are acted upon. Hence, the subject of an active verb is an agent, while that of a passive verb is the patient.