I'm working through this exercise. It has labortablo and skribotablo as two of the words. Why isn't the former laborotablo instead? Furthermore, would labora tablo and skriba tablo be equivalent, respectively?
I can't say I know for sure. In this particular case I imagine skribotablo is preferred because skribo conveys the act of writing while skrib- is for the broader concept of write, but I have seen skribtablo accepted as an alternative form. Skriba tablo on the other hand would imply that it is a 'written table' instead of a 'writing table', which sounds much less like a desk than it does some kind of sketch. I imagine difficulty/simplicity of pronunciation also play a role in which one should be used.
Edit: You can probably find out more here.
I would assume it's simply a phonological matter: you cannot easily pronounce *skribtablo because of the two successive plosives. It is of course perfectly possible, but skribotablo is somewhat easier to say.
With labortablo that's not the case, and laborotablo adds an extra syllable which is not necessary to ease pronunciation.
So, it's the principle of least effort that is at play here.
The -o is usually omitted unless its presence eases pronunciation...
- minejo de oro = gold mine
- or(o)minejo = gold mine
- forko por fojno = fork for hay
- fojn(o)forko = hay fork
- leviĝo de la suno = rising of the sun
- sun(o)leviĝo = sunrise