Sentences

 Sentences are classified according to clause structure. If it is single clause, it is a simple sentence e.g.  I read the book.  If there are 2 or more clauses, it is complex e.g. I read the book which I borrowed from the library. 

Note: a single clause will have ONE verb, 2 clauses 2 verbs - in the example of complex sentence,  two verbs: read, borrowed. Both sentences are declarative i.e. a statement is being made. The predicate is known.

Take: Who read the book? It is 1 verb, one clause, hence simple and interrogative.

 But: I would have read the book if I had been able to borrow it. 

This has 2 clauses, main clause, and subordinate clause ('if' type clause is called a conditional clause and it is always a subordinate clause, never a main clause). 

It can also be transformed into an interrogative: would I have read the book...?

Thus this is  a two fold label for sentences: simple, declarative, complex declarative, simple interrogative, complex interrogative and so on. There is a third label: active/ passive form of verb. So ':I read the book' - simple, declarative, active. 'The book was read by me'. - simple, declarative, passive.

In interrogatives, one of the elements - the subject or predicate is not known. e.g. Who read the book? ( Subject not known). What did you do?(predicate not known)

Imperative has a sentence structure in which subject is eliminated, verb is there. E.g. 'Read the book' (Implied subject: You) simple active imperative.

 'This book must be read' simple passive imperative. 'Read the book and make notes'- compound (because of coordinator 'and', 2 verbs), imperative, active. It's passive form will be ' this book must be read' and notes must be made', which retains the imperative meaning.

In grammar we use three or more levels of labelling structures according to FORM, FUNCTION, MEANING. Just as in phonetics we use three levels - place of articulation, manner of articulation and voice. Thus one sound 'b' can be classified as bilabial (place), plosive (manner) and voiced (vocal cord vibration).

 Only in this way does modern linguistics classify and describe levels of language structure.

Prof. Pushpinder Syal