I was browsing through the Wikipedia page on relative pronouns and I found this table:
| Position | With explicit relative pronoun | With omitted relative pronoun | In formal English |
|---|---|---|---|
| Subject | That?s the man [who ran away]. | ? | That?s the man [who ran away]. |
| Direct object | That?s the man [who I saw yesterday]. | That’s the man [I saw yesterday]. | That?s the man [whom I saw yesterday]. |
| Indirect object | That?s the man [who I gave the letter to]. | That?s the man [I gave the letter to]. | That?s the man [to whom I gave the letter]. |
| Oblique | That?s the man [who I was talking about]. | That?s the man [I was talking about]. | That?s the man [about whom I was talking]. |
| Genitive | That?s the man [whose sister I know]. | ? | That?s the man [whose sister I know]. |
| Obj of Comp | That?s the man [who I am taller than]. | That?s the man [I am taller than]. | That?s the man [than whom I am taller]. |
The “in formal English” column is a bit odd because it only contrasts to the “with explicit relative pronoun.”? I’m guessing the middle column is supposed to be “informal,” but it doesn’t include nonstandard forms that can omit a subject relative pronoun, like “There was a farmer had a dog” (observed by Arnold Zwicky among others), or those that can omit a genitive relative pronoun, like “That’s the guy’s name I was trying to remember.”