Lately I've had a number of different people have a number of different kinds of confusion over which female title to choose when writing in English, so I thought I'd put together a bit of a primer.
Please note that, in all cases, no matter what other factors are in play,
the stated preference of the individual being referred to takes precedence over any and all other considerations.
Ms. is the English generic, and as a general rule you should only use Mrs. or Miss if you know the person being referred to prefers that form of address.
However, people who are, for whatever reason, naturally disinclined to use Ms. usually aren't comfortable with that guideline. I've found some people's visceral response to my instruction to use Ms. is "Yeah, but..." So here are some more ways of thinking about it to determine if that "Yeah, but..." is founded.
Do you want to use Mrs.? Is the subject married? If so, is the surname with which you want to use Mrs. her husband's surname? If the answer to either of these questions is no, you
must not use Mrs. Technically, Mrs. means "wife of" and is used with the subject's husband's name. It is technically incorrect to use it with a surname other than the subject's husband's, so you may not refer to anyone as Mrs. Maidenname. Divorced women may correctly choose to use Mrs. with their ex-husband's surname, but there's too much potential for offence in calling someone who isn't married "Mrs." unless you know her preferences. Women who have a wife rather than a husband may also correctly choose use Mrs. with their wife's surname, but, again, there's too much potential for offence in introducing such patriarchal connotations unless you know her preferences.
Note that Ms. does not imply unmarriedness. It does not presume to comment on marital status.
Do you want to use Miss? Traditionally, Miss means unmarried, but it also has negative connotations for many people. It can be insulting to young women who want to be seen as mature and grown-up, and it can be insulting to older women who don't want to be thought of as spinsters. The most effective way to explain the precise flavour of the negative connotations is to think of Miss as an accusation of virginity. (Yes, this example is in poor taste, but it's by far the most effective way to explain the negative connotations to someone who doesn't grok them.) When you find yourself reaching for Miss, ask yourself: do you think the subject would want people to think that she's a virgin (regardless of whether she actually is)? If you were in her position, would you want people to think of you as a virgin? If the answer is no, you
must not use Miss. So if the subject is 12 years old, Miss is probably okay. If she's 30 years old, it would probably be a diss. If she's 18 years old, it would be rather condescending.
Note that Ms. does not imply non-virginity. It does not presume to comment on personal history.
If you're going to get it wrong, Ms. is the best way to get it wrong. Calling a woman Ms. when she prefers something else is like calling a man Mr. when he prefers something else. If it's a mistake, it's a perfectly understandable mistake. For example, suppose you meet a man you know nothing about except that his name is John Smith. So you address him as "Mr. Smith." No problems there. But it turns out Mr. Smith is actually in the military, and is properly addressed as Col. Smith. That's fine, and you'll use it in the future. But you had no way of knowing that going in, so your use of Mr. was perfectly understandable. However, suppose when you meet Col. Smith he's wearing his uniform so you can see he's in the military. But you don't know your rank insignia very well, so you end up calling him Sgt. Smith. That would be a huge diss! Or suppose you remember that he doesn't go by Mr. but don't remember what it is he does, so you take a guess and call him Dr. Smith. That would just be weird! Unless you're absolutely certain of what his actual title is, Mr. is the best way to get it wrong. Similarly, Ms. is the best way to get it wrong.
Pour les francophones: Oui, le titre féminin utilisé par défaut en français est Madame. Mais Madame, dans le sens du titre défaut, ne se traduit pas par Mrs.! Mrs. est manifestement incorrect si la personne en question n'est pas mariée ou n'utilise pas le nom de famille de son mari. Le titre défaut féminin en anglais doit être Ms.
When translating from French to English: Always always always translate Madame/Mme. as Ms., unless you specifically know the subject prefers something else.