When should com­pound words be writ­ten as one word, with hy­phens, or with spaces?

Solution 1:

In English, there are three types of compound words:

1. the closed form, in which the words are melded together, such as firefly, secondhand, softball, childlike, crosstown, redhead, keyboard, makeup, notebook;

2. the hyphenated form, such as daughter-in-law, master-at-arms, over-the-counter, six-pack, six-year-old, mass-produced;

3. and the open form, such as post office, real estate, middle class, full moon, half sister, attorney general.

For the most part, compound words that are created by adding a prefix are not hyphenated. For example, there are the words anteroom, extraordinary and coordinate. Some exceptions to this rule are (from the link above):

1. compounds in which the second element is capitalized or a number: anti-Semitic, pre-1998, post-Freudian
2. compounds which need hyphens to avoid confusion: un-ionized (as distinguished from unionized), co-op
3. compounds in which a vowel would be repeated (especially to avoid confusion): co-op, semi-independent, anti-intellectual (but reestablish, reedit)
4. compounds consisting of more than one word: (poster's note: these are phrasal adjectives) non-English-speaking, pre-Civil War
5. compounds that would be difficult to read without a hyphen: pro-life, pro-choice, co-edited

Your original example of "well-intentioned" is also explained here:

The other time we must use hyphenation is to join a word to a past participle to create a single adjective preceding the noun it modifies: "a well-intentioned plan," for example, or "a horseshoe-shaped bar."

So, why isn't nonaggression hyphenated? It can be broken into non + aggression, so it is formed by adding a basic prefix onto the noun. In doing so, it breaks none of the exceptions to the rule: "aggression" is not capitalized, hyphenating the term doesn't avoid confusion, a vowel isn't repeated, the compound only consists of 2 words, and it is perfectly readable without a hyphen.

Solution 2:

I recommend the following two articles:

Hyphenated Compound Words

Hyphenated Compounds: When and Why?

Unfortunately, the news in those articles isn't good. The distinction between hyphenated and non-hyphenated compounds is largely driven by convention, and while there are some rules of thumb, you have to be prepared for numerous exceptions and arbitrary differences between countries.

Solution 3:

I am not only a published writer, but I am also a trained court reporter and freelance proofreader/editor. Professionally, at least within the career of court reporting, reasons for hyphenating or not hyphenating certain words can vary. What follows are two reasons off the top of my head why one might see certain words (and even the same word) hyphenated one place and not another:

1. Deposition agencies can have varied, and seemingly random, protocol for how they would like certain things done — including terminal punctuation of words within brackets or parentheses, citation of legal resources or statutes and, of course, certain words they do not want to see hyphenated. It would be fair to assume that publishing companies and news reporting organizations have similar guidelines that editorial staff are told to follow as well. I believe that different industries and fields of interest have some words commonly hyphenated which may differ from the way it is done elsewhere.

   Note: the rules of punctuation in court reporting can be very different from those of standard business English, creative writing, etc. I fought myself tooth and nail (tooth-and-nail?) trying to un-learn and re-learn rules.

2. Laziness of writers (depending on what it is that you are reading), who seem to have little care if the reader understands what they are trying to say or not. It follows that then, I believe, "readability" is an extremely subjective term; maybe too subjective for some, which might cause issue.

Solution 4:

There is an historical element to hyphenation. Compound words we commonly spell unhyphenated now were once two words. Then, as they became more commonly used, they became compound, hyphenated words. Finally, when used enough, they became one word. Usage seems to determine when a hyphen is used. The guide websites cited above suggest "consult a dictionary", but my 1890 Webster's Unabridged and my 1820 copy of Encyclopedia Britannica show very different usages of the hyphen from my current OED or American Heritage. (Some words reverse this process, as the "EnglishPlus" citation suggests, when they fall out of favor: "solid-state" becomes (currently preferred) "solid state".O