Essential sentences are the ones which cannot be hidden, while the non-esential are the ones which carry extra information which can be hidden. As we see in:
Mary, who is that lady sitting right there, will talk to you in a minute.
"who is that lady sitting right there" - is a non essential clause, because it adds extra information.
We do not use " that" with non essential clauses. In this case we used "Who is..." first.
Relative Clauses
Defining clause- Specifies which person or thing we mean .
Non-Defining- contains extra information, is generally separated by commas.
That
Is normally used to inttroduce defining clauses, cannot follow a preposition, and is used instead of who in everyday speech.
Whom
Is the object form of who, so it is used in object clauses, in a formal register. Whom has to be used if it follows a preposition Eg.: To whom it may concern.(*)
(*) In everyday use, it is usual to avoid thos kind of construction. Eg. Who am i speaking to ( question mark)
Whose
Can be used both in defining and non-defining clauses. It means possession, "of whom".
Where and when
They are used both in defining and non defining clauses.
When non-defining they follow a named time or place. When defining they follow words such as time, day and moment.
Omitting the Relative pronoun is common in defining object clauses. Eg.: I've found the keys that I've been looking for.
I've founf the keys I've been looking for.
It is also possible to end a defining clause with a preposition. eg.: That is the house I used to live in.
It is also possible to reduce a verb phrase after who or which to an adjectival phrase in a defining clause. Eg: Jim was the only one of his platoon who had not been taken prisioner.
Jim was the only one of his plantoon not taken prisioner.
A non-defining clause can comment on the whole xsituation described in the main clause.
Eg.: There was nobody left on the train, which made me suspicious.
Phrases with which, such as: at which time or point, in which case, by which time, in which event; can also be used to describe the whole situation.
What- means the thing or things, "what" can be used to start clauses.
I cant belive on what you told me.
a) Whatever, whoever, whichever can also be used to start clauses.
Whoever arrives first can turn on the heating.
Clauses whithout a main verb, which have th same subject and are non dfining can contain an -ing form when:
b) Actions are happening at the same time.
Eg.: wving their scarves and shouting, the fans ran onto the pitch.
One action happening before another ( the clause then explains the reason that something is happening)
Eg.: Realising there is n one at home, I left the parcel in the shed.
c) An event which is the result of another one,
Eg: I didn1t get wet, having remembered to take my umbrella.
d) A passive construction might be expected, this is often shortned a past participle.
Eg.: Having been abandoned by his colleagues, the Minister was forced to resign.
Abandoned by his colleagues, the Minister was forced to resign.
