"The teacher taught."
Translation:La instruisto instruis.
Esperanto instru·ant·o could also be translated into English as “teacher”, but only for the lack of the better word. Those two (instru·ant·o and instru·ist·o) mean a different thing.
Suffix -ist- signifies a profession, occupation, doing something for a living; whereas -ant- is a suffix of present active participle and together with the -o noun suffix it means a person performing some action. So instru·ist·o is a “professional teacher”, someone who's job is teaching (teacher of maths in school) and instru·ant·o is a “person who teaches”, someone who happens to be teaching at the time of speaking (a friend whom you have asked for help with your maths homework).
There are two meanings of the morpheme -ist-: it can signify a profession (personon, kiu profesie, daŭre, prefere aŭ ofte sin okupas pri la afero difinita de la radiko) or a follower (supporter, subscriber) of a particular doctrine, movement (adepton aŭ subtenanton de iu teorio, skolo, doktrino).
In the second meaning -ist- words correspond to -ism- words, which are these doctrines or movements, which are being supported.