There are three types of "dashes" in English. Let's start with the shortest of them: the hyphen.
Someone once wrote that "anyone who takes the hyphen seriously will surely go mad!" ...Well, it's not quite that serious. In fact, there are some principles we can use to make those "head-scratcher" decisions about whether or not to use a hyphen (or one of the other dashes we will discuss below).
The hyphen causes most of its problems in two specific areas: compound modifiers and prefixes. Let's look at these two cases.
A compound modifier consists of two or more words that together modify a noun. Here's an example: "the state-of-the-art product." When the compound modifier comes before the word it modifies, it is usually hyphenated. However, in many compound modifiers the first word is an adverb, and many adverbs are formed by adding "ly" to an adjective (e.g., "the quick response of the Fire Department" and "the quickly spreading fire"). If a compound modifier comes before the word it modifies but the first word ends in "ly," do not hyphenate (e.g., "the perfectly engineered component"). Now for the curve ball. When a compound modifier comes after the word it modifies (i.e., after a "state-of-being" verb such as "is" or "was"), the hyphen is also not used (e.g., "the house was well built," as opposed to "the well-built house"). Here's another example: "the problem was the low-priority designation of the project..." in contrast to "the problem was that the project received a designation that was low priority."
The other difficulty with the hyphen concerns prefixes such as "multi," "anti," "non," etc. The general rule is to run the prefix together with the main word, but the Chicago Manual of Style also lists several cases in which a hyphen should be used to set off the prefix: (1) when the main word begins with a capital letter or a numeral (e.g., "non-Treasury bonds," "pre-1914"); (2) when there is a doubling of a consonant or vowel (e.g., "non-native," "meta-analysis"); and (3) when a prefix stands alone (e.g., "over- or underused"). My own additional suggestion is that you consider readability. With prefixes attached to long, complicated words, it may be better to hyphenate just so the reader can visually grasp the term more easily. Take this test: Which is easier to read at a glance—"multidimensionality" or "multi-dimensionality"? I'll let you decide—just remember to be consistent within your document.
The "en" Dash
The next-longest "dash" in English is the "en" dash. This dash and the one to follow, the "em" dash, got their names during the hand-typesetting days with lead type. The "en" dash in most typefaces (a typeface is like Times New Roman, while a "font" is actually a typeface in a specific point size and style, such as TNR 14 pt bold) was the width of the letter "N," and the next dash, the "em" dash, was the width of the letter "M." Please note that while the hyphen has numerous uses, the "en" dash has only limited applicability and should NOT be used outside its standard applications. Its primary use is in "range statements" such as "pp. 214–273" or "participants aged 30–40 years." A second usage is in tables that contain columns of numeric data such as part numbers in high-technology manufacturing: e.g., Part No. 4972–3885–9248. In this case, the "en" dash makes the numbers a little easier to read. A third usage, which is somewhat controversial, is in titles and headings. I sometimes allow the "en" dash in titles with a space before and after rather than using the "em" dash (e.g., "Community Development – Overview" rather than "Community Development—Overview," but I advise being cautious about this, and I strongly advise against using the "en" dash in text except in the situations described above.
The "em" Dash
The final "dash" is the one we normally think about when we use the term "dash"—the "em" dash. The main, and normally only, use of the "em" dash is to set off "parenthetical expressions or material"—that is, material that, like a subordinate clause, could be left out of the sentence without losing essential meaning. If the "em" dash is used, it NEVER has spaces before or after—never! A way to determine whether the situation is right for using the "em" dash to is think to yourself whether the set-off material could just as easily be enclosed in parentheses or even set off with just commas. If so—but you want to give it a little more "oomph" (i.e., importance) in the sentence, you might consider using dashes. There is a principle you can apply to make these decisions: (1) commas give the least amount of emphasis to the set-off material; (2) parentheses give it little more, but not much; and (3) "em" dashes definitely attribute more emphasis and importance to the set-off material. Here are some examples:
"My neighbor's lawn, which is not well cared for, is an eyesore."
"My neighbor's lawn (which has not been cut in weeks), represents a lack of interest in keeping up the property."
"My neighbor's lawn and garden—which are completely out of control—are a blight on the neighborhood!"
Yes, these are subtle distinctions, but like driving your car, once you get comfortable with them you'll use the right forms automatically. And I'm always here to answer questions about anything related to English usage, sentence structure, grammar, etc.