Compound words are formed by joining two or more simple words. Examples are: moonlight, nevertheless and undertake.
The meaning of a compound is not always predictable from the meanings of its component parts. For example, not every friend who is a boy is your boyfriend. The formation of a compound is called compounding.

Collective noun
A collective noun denotes a collection of individual persons or objects. Examples are: jury, army, committee, team, herd etc.
In British English a collective noun may be treated either as singular or as plural. In American English a collective noun is always treated as singular.

A compound sentence has two or more coordinate clauses (i.e., independent clauses of equal rank) joined together by a coordinating conjunction.

Study the following examples carefully:

There are three kinds of sentences and they are:

a)    The simple sentence
b)    The complex sentence
c)    The compound sentence

Study the following sentence.

He wrote a letter.