It's an impersonal verb construction. Si sa = one knows.
It is sort of like the passive voice (but it isn't). For instance I passed this question with this: "One never knows what can happen".
yeah, I also used that sentence in order not to lose a heart because I was once corrected for using "you" instead of "one" even though both uses are correct in English with similar meaning.
"Si" in this case is a formal third person pronoun. It's easier if you think of the sentence in English as "one never knows what can happen". It's third person in the sense that the conjugation of "potere" is the same as he/she/it (i.e. "puo"), but when you use the word "one" in English, it's a polite, formal way of discussing either yourself, the person you're talking to directly, or a third person not involved in the conversation. In this sense the word one can refer to many people.
If you were correct, Duo would have written the formal Si not si. The link in the first thread explains the right answer. It's an Italian idiom.
I understand all this. But it's a pity that when I gave an incorrect answer DL gave me the suggested translation "We never know what can happen", which confused me!
Leo_Maguire - thank you for this explanation - for me it is/was the best one yet. Have some lingots. Ciao.
I'm afraid Leo and you are fooling yourselves. Did you check the linked doc in the first thread? Italians and Duo capitalise the formal pronoun.
have we always got to translate these impersonal construction with 'you'? I prefer to say It is never known . . . but DL is having none of it!
Good point. "You" is an informal and colloquial way to do it, and can be misinterpreted by listeners as referring to them.
The grammatically correct usage is 'one' (singular verb) and 'they' (plural verb). "One never knows what may happen"; "they say that ..."; etc. @oktaya above says "one" is accepted here, and I've had both accepted elsewhere.
To the objection that it sounds formal or posh - well, it's supposed to! It's the si impersonale for goodness sake! Even so, it often sounds better than the passive voice.
I recall Montalbano and Livia flirting purely by tone of voice: Lui: Si può? Lei: Si può. Lui: Si può!, and off they go to bed. That doesn't translate well into either "you" or the passive.