"The digit zero is the most beautiful, is it not?"
Translation:La cifero nulo estas la plej bela, ĉu ne?
I studied Japanese for a while and that's how I remember it, actually. Ne is used in the same manner sometimes. I remember a lot of 'desu ne?' Chu is used as a word for kiss and the sound small rodents make. Thank you, Japanese. Cxu ne now appears in my head as a hamster or mouse asking the question.
My guess is that this is something that is unclear due to the English language - if I look them up over on Lernu! (lernu.net), there is a slight difference between the two, but the difference isn't really clear until I translate them to Swedish (my native language) :
Esperanto → English
nul zero, nought
nulo (nul·o ← nul) zero, null, naught, nought
Esperanto → svenska (Swedish)
nulo (nul·o ← nul) nolla
(and in German it's really clear
nulo (nul·o ← nul) Null
as German nouns have initial upper-case letters)
Basically, nul is the number (amount) in itself - zero, 0 - while nulo is the noun - _a_ zero.
The word for the number zero in Esperanto is nul or nulo.
Cardinal numbers such as one, two and three never take any endings in Esperanto.
Adding -o makes them into nouns.
1 - 1 = 0 (nul)
The number '(one) thousand' contains three zeroes: 1000.
La nombro 'mil' enhavas tri nulojn: 1000.
/disclaimer: I'm a komencanto.
Because that means "more beautiful".
So you could use it if there were exactly two digits - then zero might be "la pli bela" of the two.
But if there are many digits, and zero is more beautiful than all of them, it is "la plej bela", the most beautiful.
Unlike languages such as French or Italian, but like languages such as English or German, Esperanto distinguishes between comparative and superlative -- adding the definite article to the comparative does not make it superlative, and so "the more beautiful digit" (of two) and "the most beautiful digit" (of many) are distinct.
Yes, it is the same. There are several ways to portray the letters with diacritics in Esperanto in cases where an actual keyboard that types them is not available. Most notably are the x-system, the h-system, and the '-system. Respectively, they would be cx, ch and c' for ĉ, and similarly for the other ĉapelitaj literoj.
To build on what VincentOostelbos said, Duolingo uses the x-system as an alternative to diacritics in Esperanto for those who cannot type them on their devices.
The advantage the x-system has over the h-system is that x is not a letter used in the Esperanto alphabet, and so its use as a diacritic alternative is entirely unambiguous.