1. A machine translation method comprising: receiving a translation-source sentence in a first language via a microphone that receives a voice input performed by a user, or receiving the translation-source sentence via a text input device that receives a text input performed by the user; generating a plurality of different first forward-translated sentences resulting from translation of the received translation-source sentence into the second language; determining into which of an interrogative form, an affirmative form, a negative form, and an imperative form each of the plurality of different first forward-translated sentences is classified; determining a plurality of different second forward-translated sentences from the plurality of different first forward-translated sentences, based on the classified form; generating a plurality of backward-translated sentences resulting from backward translation of the plurality of different second forward-translated sentences into the first language; and outputting, upon receiving an operation of the user for selecting one of the plurality of backward-translated sentences during output of the plurality of backward-translated sentences on an information output device, a forward-translated sentence corresponding to the selected backward-translated sentence to the information output device.
10. The machine translation method according to claim 1 , further comprising: generating a backward-translated sentence group that is a collection of at least one backward-translated sentence generated for each of the plurality of different second forward-translated sentences; determining into which of an interrogative form, an affirmative form, a negative form, and an imperative form each of the at least one backward-translated sentence included in the backward-translated sentence group is classified; and selecting the plurality of backward-translated sentences from the backward-translated sentence group, based on the classified forms.