Connecting your clauses with a semi-colon might provide a pause that is just right, while a dash could make that pause more dramatic by highlighting the following clause, or a colon could signal that the second clause expands on the first.
Use an em dash instead of a colon when you want to emphasize the conclusion of your sentence without giving it all of the connotations that a colon brings. A dash can add “extra” information and is more flexible in this way. It is also less formal than a colon.
An em dash is a punctuation mark that can be used to replace commas, parentheses, colons, and semicolons. In general, the em dash is seen as being more interruptive or striking than other punctuation, so it is often used stylistically to draw a reader's attention to a particular bit of information.
Rule. Use a semicolon between closely related independent clauses which are not joined by a coordinating conjunction. This rule means that semicolons are used between two complete sentences which are not already linked by words like and, but, or, nor, for, so, yet.
Although they also have other uses, dashes and colons can both be used to introduce the next part of a sentence, and the difference between the two marks is subtle. A dash is a stronger and more informal mark than a colon. Think of a colon as part of the sentence that just ambles along.
Use a colon to emphasize dialogue, show titles and time, or introduce a list. Semicolons separate two independent clauses. Use an en dash for ranges and an em dash to separate non-essential information.
I ordered a cheeseburger for lunch; life's too short for counting calories. Martha has gone to the library; her sister has gone to play soccer. The examples above are each made up of two complete, grammatically correct sentences glued together.
Semicolons are most often used to connect two independent clauses (full sentences) that are related in meaning. Both the words before the semicolon and the words after it must be complete sentences that could be separated with a period. We could go fishing on Saturday. You could borrow a pole from my neighbor.
Here's when to use a semicolon:
To separate two independent clauses that are connected by a transitional phrase or conjunctive adverb. To separate two independent clauses that are connected by a coordinating conjunction if those clauses are very long or already punctuated with commas.
Dashes are also used to mark the interruption of a sentence in dialogue: Example: “Help! This horse is going too fast,” the actor yelled. “I think I am fall—.”
Let's discuss each type of dash more fully. Again, the em-dash can be used to replace parentheses, colons, and commas.
Dashes are only used in informal writing; in academic and business writing, use parentheses, commas, or colons instead. Do not put a space before or after a dash.
In practice, a colon is often used to introduce a second sentence that explains or clarifies the previous one. By contrast, a semicolon is used to indicate that two sentences are simply related to each other in some way. For example: There was only one way to defeat the dragon: we needed a magic sword.
The dash has only one use: a pair of dashes separates a strong interruption from the rest of the sentence. (A strong interruption is one which violently disrupts the flow of the sentence.)
A hyphen joins two or more words together while a dash separates words into parenthetical statements. The two are sometimes confused because they look so similar, but their usage is different. Hyphens are not separated by spaces, while a dash has a space on either side.
In informal writing, em dashes may replace commas, semicolons, colons, and parentheses to indicate added emphasis, an interruption, or an abrupt change of thought.
A dash is expressed as an "em dash" ( — ) in printing, is expressed in plain text as two hyphens ( -- ) and is used in place of a comma, colon or semicolon for greater emphasis. It denotes a major break or pause and should not be overused.
The em dash is sometimes considered a less formal equivalent of the colon and parenthesis, but in truth it's used in all kinds of writing, including the most formal—the choice of which mark to use is really a matter of personal preference. Spacing around an em dash varies.