Many writers seem to be unsure of how to use the semicolon.

A semicolon
A combination of a comma and a full stop

The semicolon looks like a combination of a full stop and a comma, and that is an indication of how it is used. Like the comma and the full stop, the semicolon separates clauses. The semicolon links two independent clauses that are closely related. In other words, we use a semicolon between two complete sentences to indicate that what they say is of equal importance. The semicolon can replace words such as and and but.

Finally, Tom found his bike behind the garage; it was rusty and the tires were flat.
This method is based on many years of research; particularly important are the findings from four studies in the heavy vehicles industry.
The potential variables are numerous; they include everything that might influence the evaluation criteria.

You cannot have a semicolon after a dependent clause beginning with words such as since, although, when, because, etc.

Since she had visited the city several times, she knew all the tourist spots.
She had visited the city several times; she knew all the tourist spots.

To avoid confusion, use a semicolon to separate groups in an enumeration.

Among the cities in the study were Hamburg, Germany; Milan, Italy; Copenhagen, Denmark; and Helsinki, Finland.

Don’t use the semicolon where you must have a colon!

He had three main interests: vintage cars, whisky and antique furniture.

You cannot write *He had three main interests; vintage cars, whisky and antique furniture (For the use of the asterisk read at the end of this text). After a semicolon there must be a complete, independent, clause with a subject and a verb. So you could write: He had three main interests; they were vintage cars, whisky and antique furniture.