When it comes to joining clauses, mistakes are almost unavoidable by learners of ESL/EFL. There are three ways of joining clauses correctly.

Joining clauses using coordinating conjunctions

One often use coordinating conjunction in joining clauses is “but”. Take a look at the following example:
Example:
Dan got married in 1998, but he was divorced by 2005.

Joining clauses using a semicolon

A semicolon (;) can be used to join two clauses when there is no connecting word.
Example:
Dan got married in 1998; he was divorced by 2005.

As you can see, the semicolon joins the two clauses. It will be wrong joining the two clauses using just a simple comma or no punctuation mark at all. The following examples are wrong:
Mark got married in 2001, he was divorced by 2003.
Mark got married in 2001 he was divorced by 2003.

Joining clauses using a conjunction or a relative pronoun

Many situations require a relative pronoun for joining two clauses. Below is the same compound sentence, but this time we use a relative pronoun for joining the clauses.
Example:
Mark, who had got married in 2001, was divorced by 2003.