Relative Clauses

A relative clause starts with a relative link which refers to the preceding noun to distinguish it from other nouns. This clause reveals the clear meaning of the noun and without it is not clear what we are talking about. 

For persons we use:

who / whom (very formal) / that, whose + noun. 

• The woman who sold you the flowers is my sister. 
• The woman who / whom you met is my sister.
• The man that served you is impolite. 
• The lady whose dress is red is my neighbour.

For thing we use:

which / that / whose + noun / of which (very formal) or that... of (less formal) + noun. 

• The computer that you bought is out-of-date. 
• The computer which you bought is out-of-date. 
• The computer whose / hardware of which you replaced does not work. 
• He has written the book of which (very formal) I have forgotten the name, or... that I have forgotten the name of.

All / everyone / everybody / no one / nobody /  those + who / that.

We omit the relative link who / whom / that / which if there is a subject-predicate construction after the link:

• The executive (whom / who / that) he employed is incompetent.

We never omit the link if there is only predicate after the link:

• The executive who / that was employed is incompetent.

Prepositions are placed before the link in formal English, in this case only whom for persons and which for things are possible:

• The businessmen with whom / to whom / about whom / several of whom / many of whom / two thirds of whom etc. 
• The pictures about which / some of which / a few of which, etc.

Prepositions are placed at the end of the clause in informal English, in this case we more often use that for persons and things and who / whom / that for persons or omit the link.

• The man (that / who / whom)
• l spoke about / to... 
• The picture (that)...
• I was presented with...

All / everything / little / much / none / no / no-compounds / superlatives + that (never which)
Relative adverbs are when / where / why: 

• The day when (on which)... 
• The place where (in / at which)... 
• The reason why (for which)...

For non-defining relative clauses used after the nouns which are already defined and are not essential in the sentence, thus can be omitted without causing confusion. The relative link can never be omitted in a non-defining relative clause:

For persons: who / whom / whose never that For things: which/ whose/of which never that

We use preposition + whom, but who + the end position for prepositions for people: 

• Mr Jones, for whom I was working, ... 
• Mr Jones, I was working for, ...

We use preposition + which or which + the end position of preposition: 

• This road, along which we are driving, ... 
• This road, which we are driving along, ...

We use comma + which for non-defining clauses referring to the whole preceding sentence: She eats a lot, which makes me irritated.

What = the thing/things that = never refers to a word /words in the preceding sentence: 

• What I like about this is the quality.

Commas are important: 

The men who..., (not all the man only those who).

The men, who...(all the men).