"His letter is good."
Translation:Sa lettre est bonne.
"Bonne" also works and is accepted. Consider that "bon/bonne" is usually an adjective and when modifying a noun it means good, suitable, efficient, correct, useful, etc. "Bien" is usually an adverb but it may also be used as an adjective (only with a linking, i.e., copular, verb, such as être) to mean good, moral, right, healthy, etc. Tricky? Yes. Here are a few examples:
- il est bon étudiant = he is a good student
- il est bien comme étudiant = he is a good student
Bien is used when you are referring to a verb - He throws well - il jete bien.
Bon/bonne are used when you are referring to a noun, which are either masculine or feminine, so the modifier (good) needs to be masc or fem also.
The wine is good...LE vin est bon.
The beer is good...LA biere est bonne.
If you are talking about something that uses le or la, un or une, masculin or feminine, you have to use bon or bonne if you want to say it is good.
I think the cause of the confusion with "bien" and "bonne" has to do with the fact that French has formalized some aspects of the language where English has not. For example, most people in normal English speech say "it's me" rather than "It is I." However, it is not accepted in formal English. French does the same thing, "c'est moi." They reclassified the pronoun into a special class, I think, but it's clearly from an accusative root. It is accepted, it seems, in formal usage. In English there is also some well/good confusion although it may sound "good" to me, it's technically incorrect. It looks as though French has formalized the same discrepancy in the opposite way: sometimes you can use an adverb as an adjective.