"Are Klingons crazy? No."
Translation:maw''a' tlhInganpu'? ghobe'.
The sound is half like an English T and half like an English L and in other languages that have the sound, it is usually transcribed into Roman letters as just tl, but in Klingon h is used to mark other multigraphs (single sounds represented by multiple Roman letters), so Dr. Okrand made it tlh (with a lower case L).
An unmarked noun might be singular or plural and tlhIngan could be interpreted as "a Klingon", "the Klingon", "the Klingons", or "Klingons", depending on the context. Adding the suffix -pu' makes it definitely plural and removes those singular versions from consideration.
The context might also restrict some of the options. The verb prefix is sometimes, though not always, limiting. The suffix -chuq can only be used with plural subjects, so would also remove the singulars from consideration. And sometimes the rest of the conversation can indicate whether singular or plural people are being considered. In this case, since there is no such context, translations with either the singular or the plural are accepted as correct answers.
The previous Skill, "Basic", which introduces the plurals, mentions that a noun can be plural even without the suffix. Since it is still a new concept, perhaps this Skill is too early to begin showing nouns without the plural suffix as plural. I have changed the structure of this exercise so that it still accepts tlhIngan without the plural as a correct answer, but shouldn't show it as the correct answer anymore (once the system updates which can take a day or two). In fact, I think you'll find that the translation shown at the top of this discussion now includes the plural marker.