A question on relative clause

Question

I couldn't understand this explanation even though I know what relative clause is:

Omission of the relative pronoun whom is particularly common when the verb is linked to a preposition. Compare the following 'defining' relative clauses:

1. 'The friend who I went out with last night is anorexic.' (Grammatically incorrect, but sometimes/often heard.)

2. 'The friend I went out with last night is going to have a baby.' (Omission of the relative pronoun would be the most common occurrence in these sentences.)

3. 'The friend with whom I went out last night has bought a new car.' (Quite improbable, almost impossible, because, as conversational English, it is far too clumsy.)

However, in this statement, which is much more formal, it is possible to link the preposition with 'whom':

4. 'The senator, with whom I dined last night, will be the next President of the United States of America.'

_{Source: http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv47.shtml}

In example of 'The friend who I went out with last night is anorexic', relative clause is defining relative clause and the relative pronoun is the object. So relative pronoun could be omitted but even if it is not but why is the first sentence grammatically incorrect?

can we say "we use whom instead of who when the relative pronoun is the object " if we can I think this is only accurate form "The friend whom I went out with last night is anorexic" ?

Answer 1

Why is the first sentence grammatically incorrect? I see two reasons. First, the preposition is stranded. That "with" is logically attached to the "who" but it doesn't appear anywhere near it. Second, if the "who" is the object of "with" then it should have the objective form "whom".

Why is the second sentence an improvement? Since the relative pronoun is omitted, it's not possible for it to have the wrong form. There's no difference between "who" and "whom" when neither one of those forms is actually present. Also, it's hard to say that the preposition's object is in the wrong place relative to the preposition when the preposition's object doesn't appear anywhere.

Why is the third sentence awkward? Well, there's no real answer to that. The grammar found in the third and fourth sentences is the same. The structure is common in the formal registers of many educated dialects, but absent in the informal registers of most conversational dialects.

Sentences three and four are both technically correct, and for exactly the same reasons. The difference is that sentence three is in a context that makes it sound inappropriately old-fashioned and condescending, while sentence four is in a context that makes it sound appropriately old-fashioned and formal.

Your proposal uses the objective form but leaves the preposition stranded. Some dialects find that acceptable, others don't. If you're going to follow these guidelines, then you'll use the pattern of sentence two in ordinary conversation and save the pattern of sentence four for special occasions.

Yes, I realize that sentence three uses a restrictive relative clause while sentence four uses a non-restrictive. That doesn't happen to be relevant to the question at hand, since the relative pronoun "whom" can be used in either case. What is relevant to the question is how the relative pronoun relates to the preposition of which it is the object.

Or, informally, the preposition it's the object of.