"Capace" means able, capable or capacious according to my trusty dictionary (and to my less trustworthy understanding!) I suspect your problem comes with the current hover-over hints that contains the word "good." These hints are becoming a little notorious for being somewhat misleading. Time will sort them out but treat them with utmost caution - I've lost many a heart over such things!
I thought I hovered and saw the 'Explain' hint that tells this is like many other words in this section that end in a/o/e/i ... yet it seems that here this singular noun (una donna) takes a plural ending (capace)? Can someone help me understand this word and clarify what endings it takes?
No, @dirty_joke. «Capace» is not a verb that is conjugated; it's not «io capace», «tu capaci», «lui/lei capace», «noi capaciamo», etc. It is an adjective; it is one of those adjectives that ends in «e» if the speaker is singular and «i» if the speaker(s) is/are plural. For example, «Tu sei capace.» but «Voi siete capaci.» Just like «Io sono intelligente.» but «Noi siamo intelligenti.»
Ah. I believe your confusion comes from the use of the confusing terminology that we, unfortunately, use. "Conjugating" generally applies only to verbs: «io vedo», «tu vedi», "I walk," "you walk," "he/she walks," etc. What you are referring to is called "agreement." You might hear something called "subject-verb agreement," which, in that case, is essentially the same as conjugating correctly, i.e. you can't say "he/she walk." Agreement with regards to adjectives and nouns, as far as Romance languages go, is just that. Adjectives and nouns should match/agree in number: «bella casa» is right, but «belle casa» is wrong. Adjectives should also agree with the nouns in gender, which is why «bel casa» and «alcune libri» are also wrong.
I hope this helped. Basically the word "conjugation" should only really be used when referring to verbs. "Agreeing" is like matching and can be used in a more general sense: subjects and verbs, adjectives and nouns, etc.