The main task of the semicolon is to mark a break that is stronger than a comma but not as final as a full stop.  It is used between two main clauses that balance each other and are too closely linked to be made into separate sentences.

  • EXAMPLE:  The road runs through a beautiful valley; the railway line follows it.

Use the semicolon to organise long, complex sentences

You can also use a semicolon as a stronger division in a complex sentence that already contains commas.

  • EXAMPLE: This week's winners are: Joe from Clare, South Australia; Diane from Perth, Western Australia; and Matt from Sydney, New South Wales.

Because each item in the list requires a comma to separate the city from the state, you have to use a semicolon to separate the items themselves.

Use semicolons to create variety

One reason you might choose to use a semicolon instead of a full stop is if you wanted to add variety to your sentence structure; for example, you might use a semicolon if you thought you had too many short, choppy sentences in a row.

Use semicolons to emphasise relatedness

Another reason to use a semicolon is to draw attention to how related your two clauses are.

  • EXAMPLE:  I have a big test tomorrow; I can't go out tonight.

The semicolon highlights that the reason you can't go out tonight is that you have a big test tomorrow. You wouldn't write, “English is my favourite food; I can't go out tonight,” because those two main clauses have nothing to do with each other.

Semicolons should not be used before conjunctions such as 'and,' 'so,' and 'but'

Avoid using a semicolon and a coordinating conjunction such as 'and', 'so', and 'but' to join two main clauses; that's the job of a comma.  

EXAMPLE:  I have a big test tomorrow, so I can't go out tonight. (The comma is used.)

Exceptions: using semicolons before 'and,' 'so,' and 'but' to organise complex sentences

Nevertheless, there are a couple of instances where it is acceptable to use a semicolon with a coordinating conjunction.

If you have a long sentence with multiple independent clauses, and some of those clauses contain internal punctuation such as a comma,  you can use a semicolon with a coordinating conjunction to make the separation between clauses more clear.

  • EXAMPLE:  If you want me to go out tonight, you need to help me with my homework first; and if you say no, I'll know that you don't really care about going out.

Since each half of that long sentence has a conditional clause that must contain a comma, it is acceptable to use a semicolon before the “and” that separates those two parts. You could make them two sentences, but you don't have to; and because they are so closely related, it makes a lot of sense to have them be together separated by the semicolon. The “and” after the semicolon is optional in this case.

Use semicolons with conjunctive adverbs: ‘however’, ‘therefore’ and ‘indeed’ …

Conjunctive adverbs are words such as 'however', ‘therefore’, and ‘indeed.’  Such words are used to show cause and effect, sequence, contrast, comparison, or other relationships.

  • EXAMPLE:  I have a big test tomorrow; therefore, I can't go out tonight. (The comma after the conjunctive adverb is optional.)

Use semicolons with transitional phrases:  'for example' or 'in other words' 

  • EXAMPLE:  I have a big test tomorrow; as a result, I can't go out tonight.


Reference

Quick and Dirty Tips. Do Things Better. 'How to use semicolons'. Retrieved from https://www.quickanddirtytips.com/education/grammar/how-to-use-semicolons.

Oxford Living Dictionaries. 'punctuation/semicolon' Retrieved from https://en.oxforddictionaries.com/punctuation/semicolon