Three dots in a row are called ellipsis and can be used to signify that part of a sentence is missing. It is mainly useful in dialogue. For example:
"I wonder…" said Harry.
The ellipsis suggests that Harry is thinking of something else.
Ellipsis can be used in the main body of your writing, where it also signifies an unfinished sentence. For example:
He wondered when the train would arrive. He had to be in Basingstoke by 4pm or else…
In this example, the ellipsis leaves the reader to imagine the consequences. Like exclamation marks, ellipses should be used sparingly. In general it is better to finish your sentences as this makes for more coherent writing.
A colon is used to introduce an item or a list of items.
Gemma has two favourite friends: Chloe and Zak.
There was only one mountain left for her to tackle: Everest.
Use the semi-colon to link two independent clauses that are connected in meaning. For example:
I can’t go out tonight; I have lots of homework.
This could equally be written as:
I can’t go out tonight. I have lots of homework.
I can’t go out tonight because I have lots of homework.
The semi-colon can be used because the two clauses are closely related.
Here are a few more examples:
Bath is a beautiful city; the architecture is stunning and the shops are diverse.
Where a list consists of a series of phrases, a semi-colon helps the reader to group information.
The guests arrived at the party: Mr Jones, from Cardiff; Asha Ali, from Manchester; Ms Cohen, from Hull.
In order to make a cake you’ll need: a large mixing bowl; finely milled flour; softened butter; caster sugar and eggs; a good strong arm to stir it all together.