a co-ordinating conjunction: This quarter our sales rates are twice lower than those of the previous month, so we have to reconsider our sales policy.

an independent marker: This quarter our sales rates are twice lower than those of the previous month; therefore we have to reconsider our sales policy.

Link the simple sentences with a semicolon, if they convey closely related ideas: The article explains the reasons of unemployment in Asian countries; it focuses on the blunders of the government in social policy.

You can also join simple sentences using one of the seven co-ordinating conjunctions in English: for, and, nor, but , or, yet , so.

E.g.: They are already divorced, but they keep on communicating with each other.

If you join the sentences with the help of independent markers, place a semi-colon before the marker and a comma after it. Though the comma is not obligatory it is better to use it:

He is doing his best to get a promotion; however, his attempts are left unnoticed by the employer.

The table below illustrates how we can use a coordinating conjunction or an independent marker for building up a compound sentence.

Meaning	Co-ordinating conjunctions	Independent markers
adding	, and	; also, ; besides, ; moreover, ; furthermore,
contrast, difference	, but , yet	; however, ; nevertheless, ; nonetheless,
consequence, conclusion	, so	; accordingly, ; consequently, ; as a result, ; therefore,
Tip	Recommended for formal writing to join shorter sentences.	Recommended for formal writing to link longer sentences.

A complex sentence comprises one main clause and one or more subordinate clauses:

Because Jupiter has an enormous size and gravity [subordinate clause], the Galileo probe steadily gained velocity as it approached the planet [main clause].--"Halo Nuclei," Scientific American

As a rule the main idea of the whole sentence is expressed in the main clause. In contrast to a compound sentence the clauses of a complex sentence are not equal. We at once see which idea is more important.