Translation:Half of the chocolate cake is enough.
I did some research on this, and it turns out you can only use "da" for unspecified things. E.g. "glaso da akvo" (a glass of (any) water) is possible, but "glaso da tiu akvo" (a glass of that (specific) water) is not possible. The same applies to "la akvo (the water), "via akvo" (your water), etc. In such cases you should use "de" (belonging to) or "el" (out of).
Duono da ĉokolada kuko estas sufiĉa.
Half of a chocolate cake is enough.
(It could be any chocolate cake)
is correct, but when talking about a specific cake you should say:
Duono de la ĉokolada kuko estas sufiĉa.
Half of the chocolate cake is enough.
Duono el la ĉokolada kuko estas sufiĉa.
A half from the chocolate cake is enough.
It's also worth nothing that
"glaso da vino" means a glassful of wine,
"glaso de vino" means a wineglass (a glass belonging to (any) wine), and
"glaso de tiu vino" means a glassful of this wine (a glass belonging to this wine)
If you are up for it, here is a more detailed explanation of the various rules concerning the use of "da", written in simple Esperanto. Or well, as far as texts about grammar go.
The really hard part of learning any language is mastering the usually arbitrary and confusing conceptual metaphors that we don't even think we are thinking of as metaphors in our native language. These little words and phrases: from, of, out, out of, in, into, since, when, ever, whenever, then, this, that, during, and so on (there are so many!) seem like they should be used the same way by everyone until you actually think about what you are literally saying and then you realize that no one metaphor really captures what it is you are trying to convey.
Money walks in and out of my wallet. Money goes in and out. The money is flying out of my account. The money crawls around Wall Street. Money is creeping into every facet of our lives. Money slinked its way into the conversation. I could go on endlessly with these types of metaphors. And if I were to translate them literally into Esperanto, depending upon whom I am addressing, they could know exactly what I mean in all its nuance, or be left with a mental image of literally flying money. And they may want to address the same topics with a wide variety of flowing/non-flowing water metaphors that I may or may not understand. Money flows in and out of my wallet. Money flows in and out. The money is gushing out of my account. The money seeps around Wall Street. Money is leaking into every facet of our lives. Money leaked its way into our conversation.
And who knows what some other culture uses as a metaphor for money? Pounding on rocks? Soaring with birds? Throwing nets into the surf?
And, also, you start to get an ear for the weird metaphors that you use all the time without realizing it. You start to see that you are pushing will, intention and agency on inanimate objects, and stripping it from sentient objects constantly and automatically. It's almost a miracle that anyone can convey what they mean ever.
See RaizinM's reply. It's a fine point and for sure one I would file under "advanced" Esperanto. The course authors of FEC (the 10 lesson course) got it wrong, so don't feel bad.
To piggyback on RaizinM's explanation, it's not that strange that Esperanto makes a distinction between quantities of definite and indefinite nouns. English also has something similar. We say "some chocolate cake" but "some OF THAT chocolate cake." When you use a word like 'the" or "that", we suddenly need a word like "of" to connect them.
The word "da" is used to connect words of measurement with the kinds of things you measure. Kilogramo da sukero. Iom da akvo. Miloj da personoj. However, as in English, if you use a word like "la" or "tiu" you suddenly need a word like "de" (of/from) because it's no longer just a measurement - but you're saying that the "duono" in question came from a particular cake.
And more recently, I wrote a whole blog post about de and da if you need more detail.
Yes, you can say "duona ĉokolada kuko", but the meaning is a little different.
- "duona kuko" and "duono da kuko" mean "half a cake" (of any cake)
- "duono de la kuko" means "half of the cake" (a half of a specific cake)
- "la duona kuko" means "the half cake" (a specific cake of which there is half left)