The dash is represented by two hyphens with no space
around them (--).
A dash may be used:
To separate two clauses when the second amplifies or
restates the first
The shirt was very cheap-- the cost was ten dollars.
When the enclosed element contains internal commas
The devices--processor, modem, hard disk, and cards--are
the essential elements in this computer.
Before a summary of preceding details in a sentence
Processor, hard disk, and modem--These are the typical devices
he needs to change.
To separates a group of antecedents from their pronoun
that is the subject of the sentence.
Car, plane, train--these are the possible means of
transportation in this town.
To enclose elements those need emphasis.
We are--human being--greedy.
To show a sudden change in structure of a sentence
He failed the exam--the final rectification will be held
soon--which the first question was about how to know
Make an appositive more emphatic
There is a simply way to have a success-- the knowledge.
Enclose a complete sentence that interrupts another
The car that we have seen yesterday costs a lot -is made in
In this case, (- is an en dash, not a hyphen. A dash is called
also an em dash)
Commas are insufficient to enclose an interrupting sentence;
dashes or parentheses are necessary.
To connect a unit modifier with a two-word or hyphenated elements
Saint John--Vancouver distance
To separate, in list, the item from a descriptive statement
They were supposed to bring:
1- Flour--Four pounds in one box.
2- Milk--The fresh one.
3- Eggs--Big, small and medium.
To Indicate inclusive or continuing numbers or dates
Read all the pages: 10--19
But it is not correct with from . . . to . . . or a
between . . . and . . . construction.
To show vague or unlimited dates
600 to 199--
To separate a title and subtitle
Prepositions with time--How to use preposition-- have
To emphasize an explanation (more than a comma or a
semicolon) with the words: namely, that is and for example
They are many strong words in her email--for example, I hate
to be discovered.
Commas indicate only a little separation within the sentence.
Dashes emphasize the element enclosed and clarify its meaning
when It contains internal commas.
Parentheses indicate that the enclosed element is only freely
connected to the rest of the sentence.