Who and whom? The usage of relative clause

Question

I'm an English learner.

I wanted to know which one is correct.

I like the girl who lives next door.

I like the girl whom lives next door.

For me I know I can separate the sentence into two sentences. Such as

I like the girl.(the girl is an object)

The girl lives next door.(the girl is a subject)

I think

I like the girl who lives next door

This one is the correct answer.

But someone tells me

I like the girl whom lives next door

It's also correct in old-fashioned English.

But for me,

I think the old-fashioned English is only used in the below situation.

I like the girl.(the girl is an object)

You met the girl last night.(the girl is an object.)

I like the girl whom you met last night. I like the girl who you met last night.

Both of them are correct.

When "the girl" is an object in the second sentence. It can meet the old-fashioned English rule.

But when "the girl" is a subject in the second sentence. It can't meet the old-fashioned English rule.

So I wanted to know if the sentence, I like the girl whom lives next door, is the correct sentence?

Answer 1

[1] I like the girl [who lives next door].

[2] I like the girl [whom you met ___ last night].

The answer, as always, lies in the function of the relativised element, i.e. the relative pronoun.

In [1] the relativised element functions as subject of the bracketed relative clause, and hence it must be nominative case "who".

But in [2] it functions as object of the verb "met", and hence accusative case "whom" would traditionally, or more formally, be the correct form. However, there is a trend nowadays to use "who" instead of "whom" wherever possible, so "who" is the preferred choice in this instance.

Answer 2

Whom would never have been correct here, even in what you call 'old-fashioned English'. Who replaces she as subject of the clause 'she lives next door'.

You are right to say that 'I like the girl whom I met last night' is correct in formal English, as here the girl is the object of the verb met.