If you’re the betting type, and you wager on whether a given word beginning with a prefix is attached directly to the root word or linked with a hyphen, bet against the hyphen: The trend—in American English, at least—is to close prefixed words and compound words. However, you won’t always win, because there are exceptions, even among words beginning with a particular prefix. Take the prefix co-, for example.
Using the Merriam-Webster’s website as the authority, we can see that virtually every word beginning with the prefix is closed. Exceptions include most words in which the root word begins with o, including co-official, co-organizer, and co-owner. Co-op takes a hyphen when it serves as an abbreviation for cooperative, even though the full term is not hyphenated—though it, like many other words containing prefixes, once was. (However, the unrelated term coop, referring to a shelter for birds or other animals, has no hyphen.)
In British English, which is generally more conservative about orthography and word treatment, the correct form is co-operative (and co-operate and co-operation). Co-opt, however, is standard in both dominant forms of English. An all-but-obsolete treatment of such words to signal that the o’s are separated by a syllabic break—this style quaintly persists in the pages of the New Yorker—is the inclusion of a diaresis (two dots) over the second instance.
Avoid attempting to attach the prefix to a phrase, as in “co-personal assistant,” which fails because it describes an assistant who is co-personal, not a personal assistant who shares responsibility with another person holding that title. A natural solution is to employ a slightly sturdier en dash in place of the hyphen to convey the prefix’s relationship to the entire phrase, not just the first word (equivalent to the stronger symbol’s usage in such phrases as “pre–Industrial Revolution”), but this strategy is not standard; instead, merely substitute co- with fellow.
Finally, avoid the prefix altogether if it is always redundant, as in copartner, and consider doing so if, in context, it is often so, as in co-conspirator.
Maybe I have learned something new tonight about an old usage in English that is now obsolete**. Also, I had never heard of it to begin with, and it might as well have remained an unknown quantity:
“this style quaintly persists in the pages of the New Yorker—is the inclusion of a diaresis (two dots) over the second instance.”
“Diaresis”? I’ve always perceived the double-dotting over the letters “a”, “o”, and “u” as being called “umlauts”. Umlauts are common in German, Danish, Swedish, and Norwegian, and maybe less so in Dutch and Flemish. (In a few lesser-known languages, umlauts are even used over the letters “e”, “i”, and “y”, but those are rarely seen or noticed.)
There is an interesting twist about umlauts over capitol letters in the German language. This is done in Germany and Austria like so: Ä, Ö, Ü, but this is not popular in Swiss-German. The difference is often seen in signs, such as street signs and signs at train stations and airline terminals, and especially when the letters are in all-capitals and/or written in a vertical column. For example, the official name of Austria in the German language is Österreich, or ÖSTERREICH in all-caps, but in a Swiss railroad station or airline terminal, you are more likely to see OESTERREICH, or even better on a vertical sign.
By the way, historically, Austria has been mostly east and southeast of Germany, and especially back before November 1918, when the Austro-Hungarian Empire included Czechoslovakia and a large slice of what is now Poland. (Well-before 1914, Poland had vanished as a country because it was repeatedly dismembered by its neighbors Germany/Prussia, the Russian Empire, and the Austro-Hungarian Empire.) The very meaning of the word “Österreich” is “eastern kingdom” because from the point-of-view of the German Empire, Österreich was to the east and southeast.
**Beware the difference between “obsolete” and “obsolescent”. “Obsolete” refers to something that is all the way out of common use, and “obsolescent” refers to something that is on its way out. In warfare, obsolescent warplanes will soon be shot down, unless they are transferred to something like training duties, in which case they are not warplanes anymore.
Obsolescent warplanes were a big problem for many countries during the years around 1939-42, opening up World War II. Obsolescent fighter planes were a huge problem for the air forces Belgium, China, Denmark, France, Italy, Norway, Poland, the USSR, and the United States, though the latter was fortunate to have the better P-38s and P-47s under development and the industrial capacity to make them with. The British RAF had obsolescent bombers in the Blenheim and the Whitley, and the Royal Navy has obsolete fighters, torpedo places, and dive bombers for its aircraft carriers. The Allied solution was for the British to buy carrier-based aircraft from the Americans, all of the Allies to get patrol planes like the B-24 from the Americans, and the Australians, New Zealanders, and the Dutch to get warplanes from the Americans. Where was the Dutch air force doing anything? It used American planes from bases in Australia to fight the Japanese in the Dutch East Indies (Indonesia). The Soviet Air Force also got large numbers of fighter planes from American factories, delivered by flying them via Canada, Alaska, and Siberia.
By 1944, the Japanese were flying obsolescent warplanes against the Americans, Australians, New Zealanders, British, and Dutch, and things were getting worse and worse for the Luftwaffe because of the vastly-improved American P-51 Mustangs and the new warplanes that the RCAF got from the Bristish.