Pero vs sino
Is there any rule about which form of "but" to use and when? I can't figure out any pattern.
That is probably worth a Google "pero versus sino SPanish" rather than an awkward/wordy explanation (mine would be that way) here.
Duolingo seems very weak on this sort of thing, it should really either call out the relationship between words like this or better yet let us practice their use directly (e.g. Give us a sentence and let us choose which to use).
Sino is an elegant way to provide a non-obvious alternative to a negative statement, without having to start a new sentence.
To avoid confusing it with pero, try replacing it with although–if it works, it's pero; otherwise, it's sino.
> — Me apetecía verte hoy, pero hasta mañana no puedo.
> "I felt like seeing you today, (but/although) until tomorrow, I can't."
> — Pues yo no contaba con verte hoy, sino mañana, así que perfecto.
> "(Well) I wasn't counting on seeing you today, (but/I was counting on seeing you) tomorrow, so that's perfect."
I go deep into the usage context of sino in this NachoTime post.