"Yes, birds eat insects, because they are delicious."
Translation:Jes, birdoj manĝas insektojn, ĉar ili estas bongustaj.
Both are fine here.
Bongusta = good tasting, delicious
Bongustega = very good tasting, delicious, very delicious
Adding the -eg- affix makes the property (in this case tasting good) even stronger.
You can use it in many many words. Another example: granda = big, while grandega = huge
At first I thought, "even with the availability of an accusative case, this lesson's sentence requires a subjective decision about whether the birds or the flies are tasty." Then I realized that the clause "ĉar ili estas bongustaj" can't have an accusative case, because "estas" doesn't take an accusative.
Jes kaj ne. All accepted alternatives have to be keyed in, so it is very likely that an alternative with "bongustegas" isn't among them. I can see two reasons for this:
1. there isn't any amplifier in the English text
The -eg- in bongustegas is an amplifier, that should be expressed somehow in the English text, e.g. "very/really delicious". Now the Esperanto text is always the original one in Duolingo, so absence of "very/really" in the English text confirms, that absence of -eg- is intended.
2. verbified adjectives vs. esti + adjective
While you can verbify any adjective or noun in Esperanto (e.g. bongusta → bongusti; martelo → marteli) you should ask yourself, whether you should do that. For adjectives verbification works in few cases, mostly not. There are a couple of established expressions (pravas, malsatas, gravas etc.). Bongusti is a border-case. For more info see Esperanto Language blog by Salivanto.
Thank you for the answer!
Ad 1. My impression was that "delicious" is more delicious than just tasty, which, as I understand, translates to "bongusta". Maybe it is rater a nuance of English (also not my native) than Esperanto :)
Ad 2. Thank you for the explanation and the link. Up to this point, I thought that "estas X-a" is fully interchangeable with "X-as".
Why is using the accusative ending on "ili" wrong in this translation: "Jes, birdoj manĝas insectojn ĉar ilin estas bongustaj." "Ili" is referring to the insects (insectoj) which received the accusative ending and "ili" is not part of a propositional phrase, so should it not also receive the accusative suffix?
It is because "Jes, birdoj manĝas insectojn ĉar ili estas bongustaj." is in effect two sentences linked by the conjunction "ĉar". "Birdoj" is the subject of the first section, and "ili" is the subject of the second section. Take a simpler (but similar) sentence: "Paŭlo batas Petron ĉar Petro batis lin" (Paul hits Peter because Peter hit him). There, "Paŭlo" is the one who does the action in the first section, so "Paŭlo" is the subject. It's the other way round in the second section - Petro has done the hitting there, so he is the subject. I hope that makes things a bit clearer for you.
I'm not sure what you mean by compound and complex-compound sentences, but I would like to make a slight correction to what David Lamb said. The exercise has one sentence with three parts:
- Jes : a responding word
- birdoj manĝas insektojn : the main clause
- ĉar ili estas bongustaj : the subclause
To be a clause a predicate verb, ĉefverbo, is required in Esperanto. Even if the subclause here is a subordinate clause (as David pointed out the conjunction ĉar combines the clauses), the requirement is still there.
Most of the verbs in Esperanto also require a subject (not all as in English). Both manĝi and esti are such. In the main clause the subject is birds, and in the subclause something that refers to the insects.
(A different question is that to my ears that referring word should be tiuj, because I associate it with the object in the main clause, i.e. insektojn, whereas ili gets associated with the subject of the main clause, i.e. birdoj. Thus the birds would be tasty.)