Clause structure

Clause structure.

All clauses in English have at least two parts: a noun phrase and a verb phrase: 

Noun phrase (subject)	Verb phrase
The children All the people in the bus	laughed were watching

But most clauses have more than two parts:

Noun phrase (subject)	Verb phrase
The children John All of the girls This soup Mary and the family She	laughed wanted are learning tastes were driving put	a new bicycle English awful to Madrid the flowers	in a vase

The first noun phrase is the subject of the sentence:

The children laughed.
John wanted a new bicycle.
All the girls are learning English.
She put the flowers in the vase.

English clauses always have a subject:

His father has just retired. Was a teacher. He was a teacher.
I’m waiting for my wife. Is late. She is late.

… except for the imperative which is used to give orders:

Stop!
Go away.

… and for "soft imperatives" like invitations and requests:

Please come to dinner tomorrow.
Play it again please.

If we have no other subject we use "there" or "it" as subject. We call this a ‘dummy subject’:

There were twenty people at the meeting..
There will be an eclipse of the moon tonight.

It’s a lovely day.
It’s nearly one o’clock.
I have toothache. It hurts a lot.

Source: http://learnenglish.britishcouncil.org/en/english-grammar/clause-phrase-and-sentence/clause-structure