Here is a helpful table of rules for using the dash or hyphen:

The humble dash - you can find it right here - is both helpful and versatile. You can think of it as being used for interruptions.

We can use it to break up sentences in order to introduce information that we feel can't wait until later! When we are writing - whether in a personal, professional, or academic context - the key to good communication is to be concise. So, sometimes, instead of writing a lot of short sentences, we can integrate the information into a shorter number of sentences. This is called interjecting.

DASHES - INTERJECTING

As you can see from these two opening paragraphs, an interjection is a way of 'butting in' to conveniently provide extra (or supplemental) information without slowing down the overall reading. Here is what the opening sentence above would look like without dashes:

The humble dash you can find it right here is both helpful and versatile.

This sentence not only has insufficient punctuation, but it makes no sense. Without punctuation, a sentence like this could mistakenly be read without any pauses. This would prevent us from understanding it correctly. It is possible that the sentence needs commas:

The humble dash, you can find it right here, is both helpful and versatile.

As you can see, the sentence is still grammatically incorrect (and doesn't even work logically). The dashes help us to understand that there are two separate pieces of information here, and that the information between the dashes is logically part of its own sentence:

The humble dash - you can find it right here - is both helpful and versatile.

The interjection does not belong in the grammatical flow of the sentence, but the dashes indicate that it has a 'life' of its own. This is a great way of being concise, by merging sentences that would otherwise be separate. The same applies to the example below (taken from the second paragraph, above):

Here are the two sentences that exist separately:

1. When we are writing, the key to good communication is to be concise.
2. This can refer to a personal, professional, or academic context.

This is the convenient and concise merging of these two sentences:

When we are writing - whether in a personal, professional, or academic context - the key to good communication is to be concise.

Note that the comma from sentence 1 is no longer needed in the merged sentence, as the interjection replaces the pause.

DASHES - POINTING TO A FACT

The dash can also be used to introduce or 'point' to a fact or some extra information following the logical end of a sentence. Again, this means we can be concise and that an extra sentence isn't needed, but it can also help us to shorten existing sentences. Take a look at this sentence:

He looked in the mirror and smiled, feeling that he was ready for his interview.

In the sentence below, the dash points to the reason the man smiled, replacing the words 'feeling that':

He looked in the mirror and smiled - he was ready for his interview.

As you can see, the sentence is now shorter and sharper, and the dash indicates that a question is being answered before it is even asked.

DASHES - INTERRUPTING OR STOPPING ABRUPTLY

In informal writing (fiction writing, for example), a dash is usually used to show that a person is cut off or stopped abruptly. In a 'first-person' narrative - where you are writing about yourself or your character is telling the story - you might want to leave a thought hanging, like this:

I had always planned to travel the world, but...

However, if you want to convey a sharper sense of interruption, this would be the preferred form:

I had always planned to travel the world, but - 

The dash creates a sense of urgency. While this method can be used when your writing is in a 'first-person' narrative, it is more suited to dialogue:

"I had always planned to travel the world, but - "

This method in dialogue - in a screenplay or a novel, for example - is used when the next speaker, or even an action, interrupts immediately so that the person is stopped from finishing their sentence.

*Note on Length and Spacing

The dash (some call it an 'em' dash) should always be longer than a hyphen, and it is usually preferred to have a space on either side of it. In most wordprocessing programs, the hyphen will be changed into a dash (made longer) when you have a space on either side of it. The hyphen, of course, has no space, joining words closely together.

HYPHENS - BRINGING THINGS TOGETHER

Hyphens help bring things together. In compound words, words that otherwise have a life of their own are brought together to create a single word.

In this example, the introduction of a hyphen brings the words 'hands' and 'on' together to create an adjective, describing a noun.

He had a very hands-on approach. or His approach was very hands-on.