We can classify sentences according to their structure:
1. Simple: those which have only one finite (or conjugated verb).
Tigers are carnivorous.
2. Compound: those in which the constituent clauses are grammatically co-ordinate, no one being dependent on the others, but all being added together in sequence. A compound sentence will be made up of at least two main clauses.
[(I saw him yesterday) and (I shall be seeing him again tomorrow.)]
3. Complex: they are like simple sentences in that they consist of only one main clause, but unlike simple sentences they have one or more subordinate clauses that are grammatically dependent upon the main clause and that function as an element of the sentence.
[(Although I admire her reasoning), I reject her conclusions.]
[The man (who came yesterday) is my uncle.]
4.Compound-complex: they are made up of at least two main clauses which in turn contain subordinate clauses.
[(He came yesterday) and (<as you were not in>, he left this book for you.)]