Semicolons

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.

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.

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 why 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.

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.

Please note: if the emphasis is on the second clause (in other words, if the first clause is followed by a reason or explanation), use a colon!

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.

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.

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


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