Semicolons and colons and colons can be used in two ways:
To help with listing (although colons and semicolons do slightly different things for listing)
To join independent clauses to better show relationships between ideas and to ‘increase the pace’ of writing. Both do this in a similar way, but colons have a slightly more specific use.*
Extra information:
In American English, when there is an independent clause after the colon or semicolon the word immediately following the colon or semicolon is capitalised. But in British English it isn’t.
We follow the British convention, i.e. no capitalisation after the colon or semicolon (unless it’s a proper noun, in which case we must).
There are two ways we use semicolons:
To join two independent clauses to show that the ideas in both are closely connected, e.g.
The crocodile’s jaws opened wide; I had never seen so many teeth together in one place at one time.
I stayed crouched on top of the car; my brother was hiding in the recycling wheelie bin.
Whenever a semicolon is used in this way, it could always be replaced with a full stop or coordinating conjunction (FANBOYS), e.g.
I stayed crouched on top of the car. My brother was hiding in the recycling wheelie bin.
I stayed crouched on top of the car and / yet / but my brother was hiding in the recycling wheelie bin.
The advantage of using a semicolon over a full stop or coordinating conjunction is that it makes the two ideas (independent clauses) seem more closely connected and it ‘speeds’ up the writing, which communicates a sense of excitement, happiness, urgency, nervous, fear, or other emotion.
To separate items in a ‘complex’ list that has internal punctuation, i.e. to show significant groups or hierarchies between those things, e.g.
The crocodile hunting me is clearly from Brisbane, Australia; Johannesburg, South Africa; or Disney World, Florida.
So far, this crocodile has eaten my cat, two years old; my UE Boom speakers, three months old; our recycling wheelie bin, which must’ve tasted awful; and my brother, who can’t run as fast as me.
There are two main ways we use colons.
In both of these instances, it is a way of showing that what follows the colon is explaining, confirming, or clarifying what comes before the colon, i.e. showing that we’re moving from the general to the specific. The two times are:
To introduce a specific list, e.g.
There are three types of crocodiles in this world: hungry, angry, and hangry ones.
We have two options: run like mad, or push our brother in front of the crocodile.
To join two independent clauses in a way that shows that the idea in the second clause is very closely connected to the first one and that we particularly want to emphasise the second idea e.g.
I am sure of one thing: only I or the crocodile will be alive tomorrow.
In the end, I escaped the crocodile easily: your ninja stealth skills workshop was excellent.
When a colon is used in this way, it could always be replaced with either a full stop, subordinating conjunction, or phrase.
I am sure of one thing. Only I or the crocodile will be alive tomorrow.
I am sure of one thing and that is that only I or the crocodile will be alive tomorrow.
In the end, I escaped the crocodile easily because your ninja stealth skills workshop was excellent.
The advantage of using colon in these types of situations is that it more clearly makes the two ideas (independent clauses) seem more closely connected. It also reduces the ‘clunkiness’ that sometimes happens in writing whilst also speeding up the writing.