Relative Clause Removal of "Relative Pronoun"

Question

So I have a grammar book that says you can remove relative pronoun when it is used in defining relative clause, if it is non-defining you can't.

For example:

The man who we met yesterday was a sales representative.

The man we met yesterday was a sales representative.

both correct. But:

My father, who you met yesterday, lives in Germany.

According to the book you can't remove "who" from this sentence. Is this acceptable for all cases or just particular ones?

Answer 1

The book has it right, except for one further condition: you cannot remove the relative pronoun when it functions as the subject of the relative clause.

So:

The man [who] we met yesterday... - "who" is optional. But

The man who came to see us yesterday... - "who" is required.

Answer 2

The rule you cite is mostly true, but it's not complete, because just mentioning "relative pronouns" doesn't distinguish between

1. wh-words like who and which, on the one hand, which can occur with all relative clauses,
   and

2. that, which can occur only on restrictive (integrated, defining) relative clauses,

• The man whom we met yesterday was a sales representative.
• The man who we met yesterday was a sales representative.
• The man that we met yesterday was a sales representative.
• The man Ø we met yesterday was a sales representative.

But that can't occur on non-restrictive (supplementary, non-defining) relative clauses,
(in what follows, ungrammatical English sentences are *marked with asterisks)

• The chairman, who(m) we met yesterday, was once a sales representative.
• The chairman, who we met yesterday, was once a sales representative.
• *The chairman, that we met yesterday, was once a sales representative.
• *The chairman, Ø we met yesterday, was once a sales representative.

Non-restrictive relative clauses also bar relative pronoun deletion,
as the last ungrammatical sentence shows.

The rule also doesn't distinguish between most relative pronouns and those relative pronouns -- either that or wh-word -- that are the subject of their relative clause. Subject relative pronouns are also immune to deletion; English tensed clauses require a subject, and this requirement overrides.

• The man who met us yesterday was a sales representative.
• The man that met us yesterday was a sales representative.
• *The man Ø met us yesterday was a sales representative.