The smallest meaningful unit in language is the word. Words combine to form phrases, clauses and sentences.
• a sentence consists of one or more clauses
• a clause consists of one or more phrases
• a phrase consists of one or more words.
Look at these examples
(i) Nature is kind to her slaves.
(ii) As we must eat we must first provide food.
(iii) You are all young, but I don’t think you are too young to be aware of this.
In example (i) you find only one verb, is. There is only one idea expressed. It is a single clause sentence known as a simple sentence.
In example (ii) you find two sets of verbs, must eat and must provide. It is a two clause sentence. (a) As we must eat (b) We must first provide food. You can see that (b) is complete in its sense. This is the main clause. The meaning of clause (a) depends on (b). This is the subordinate clause. Sentences with a main clause and one or more subordinate clauses are complex sentences.
In example (iii) you again find two verbs: are and are (a) You are all young. (b) But I don’t think you are too young to be aware of this. In this case (a) and (b) both make sense independent of each other though there is a link. There are two main clauses joined by the conjunction but. Sentences with more than one main clause are called compound sentences.
When sentences are too long and complicated, it is useful to look for the main clause which carries the main idea and the subordinate clauses which carry ideas that depend on the idea expressed in the main clause.