A clause is the basic unit of grammar. Typically a main clause is made up of a subject (s) (a noun phrase) and a verb phrase (v). Sometimes the verb phrase is followed by other elements, e.g objects (o), complements (c), adjuncts (ad). These other elements are sometimes essential to complete the meaning of the clause:
[S]Jo [V]doesn’t feel [C]well.
Jo doesn’t feel. (well is essential because it completes the meaning of feel.)
[S]They [V]haven’t posted [O]all the invitations. (post is a transitive verb which needs an object, all the invitations)
The underlined words are not essential to complete the clause:
[S]I[V]’ll call [O]you [AD]later.
[S]All the girls [V]laughed [AD]loudly.
When we give a command, we don’t usually use a subject:
When we do use the subject, it is to reinforce the instruction or to make clear exactly who the speaker is talking to:
You be careful.
Main (independent) clauses and subordinate (dependent) clauses
Main (or independent) clauses can form sentences on their own. They aren’t dependent on other clauses. They are always finite (they must contain a verb which shows tense).
Subordinate (or dependent) clauses cannot form sentences on their own. They are dependent on main clauses to form sentences. They can be finite or non-finite (the main clauses are in bold; the subordinate clauses are underlined):
I didn’t go to work because I wasn’t feeling very well.
He studied violin and mathematics before taking a medical degree and doing postgraduate work in biophysics at Harvard.
She had pretty hair and must have been nice-looking when she was young.
If I tell him will he be angry?
We can combine clauses of the same grammatical type to form sentences using coordinating conjunctions:
[main clause]I’ll take the train and [main clause]you can take the car.
I’ll give you a call [subordinate clause]if I’m going to be late or [subordinate clause]if I’m not coming.
You can use the phone [non-finite clause]to receive calls but [non-finite clause]not to make them.
We don’t create coordinated clauses with clauses of a different grammatical type. For example we cannot coordinate a main clause and a subordinate clause:
Ten minutes passed and no one had come.
Ten minutes passed and if no-one had come.