"Esperanto is a good language."
Translation:Esperanto estas bona lingvo.
Although it's may be a nice rule of thumb that's not a matter of being planned nor dead. And it's definitely not an exception. :)
There's a general rule in Esperanto, that you don't use the definite pronoun la with proper nouns. So you just say Germanujo and not ~la Germanujo~, you just say Marso and not ~la Marso~, you just say latino and not ~la latino~. So when you call Esperanto using a proper noun esperanto, you say just that and not ~la esperanto~.
Most languages don't have those nominal names (formed by attaching -o to the root), since most of them are connected to a nation or a country, which is the base meaning of the root. So anglo means “Englishman”, Anglujo means “England” and angla is an adjective “English”. When you want to speak about a language, you say la angla lingvo (lit. “the English language”), but most often the lingvo is omitted. This nation doesn't have to have a separate country, of course, as is the case of koso meaning “Xhosa (man)” and la kosa meaning “Xhosa (language)”. The root doesn't have to mean a nationality as is the case of Sumero meaning “Sumer” and la sumera meaning “Sumerian language”.
Some languages, however, are not connected to any nationality nor country and therefore their names can be formed by just attaching the nominal ending -o. Most notably that's the case of many dead languages (latino, sanskrito, but not la prahindoeŭropa, since there is Eŭropo, and not la sumera mentioned above) and all constructed languages (esperanto, volapuko). But this type of language can also very well be natural and living, as is the case of urduo meaning “Urdu (language)” (though some would say that's the same language as la hinda meaning “Hindi”).
But with those root-type languages you may also use the adjectival form and say la latina, la sanskrita or la urdua. In my experience, that's rather frequent. Saying la esperanta would be correct and understandable, although it's hardly ever used. :) But if you really want to call Esperanto with an adjective, then why not to use its original name, under which it was published by L. L. Zamenhof — la lingvo internacia. :)
To be correct, when using the adjective esperanta you should also use the article la. So La esperanta lingvo estas bona would be a grammatically correct sentence.
But I imagine that the authors of this course have decided not to allow this very unusual version of the sentence for pedagogical reasons. ;)
This has been answered elsewhere in this thread. Basically, some languages have their own names - Esperanto, latino, sanskrito... these don't use "la". Most languages are described by adjectives - the English language, the French language, the German language. In Esperanto this is la angla lingvo, la franca lingvo, la germana lingvo - and from context we know it's a language so we don't bother to say "lingvo"
- la angla
- la franca
- la germana