Translation:How can he know so much about fish?
That would be some other sentence. The "fare a" construction is important: http://tutorino.ca/grammatica/2007/9/27/having-people-do-things-il-fare-causativo.html
"How does he come to know so much about fish." or "How does he happen to know so much about fish."
That would be a natural English translation. I have reported it because it sounds better and is still faithful to the original Italian.
Come in this context is like happen. It's not venire, it's the secondary English meaning of come.
'how does he do to know' = nonsense in English. "What dos he do to know so much" is closer to acceptable, but it takes time to build up a knowledge of something so it would be much better to talk about 'how DID he become so knowledgeable', or 'how did he get to know so much'. I have no idea what the name is for this particuar construction but it makes more sense in English.
Be careful! English has do-support, which doesn't exist in Italian. "Does he know" is just the interrogative version of "he knows". Instead, "fare a" in this and similar Italian expressions means something like "manage to". In this case "fa a sapere" means "(he/she) manages to know". On your second question I think it is fine.
- Come fa a sapere così tanto sui pesci? = How does he manage to know so much about fish?
- Perché fa a sapere così tanto sui pesci? = Why does he manage to know so much about fish?
how does he know = come sai (which it is not the same as "come fa a sapere") how can he know = come puoi lui sapere (which it is not the same as "come fa a sapere")
The problem is that sometimes DL demand a literal translation, and other times to elaborate it according the the other language. And I find that very confusing. It would help some kind of consistency.
It seems you have troubles with the conjugations.
How does he know = Come fa a sapere. fai is second person singular.
Fare is conjugated as follows:
Come puoi lui sapere is incorrect. Come fa a sapere [...] is one proper and correct translation for "how can he know [...]"
sorry, I made the mistake by changing the 3rd to a 2ond person. But my argument still is alright.
Fare = to do not Can (where "can" in Italian is "potere" and not "fare") Sapere = To know
I am spanish speaker and the Italian sentence is similar in spanish "como haces para saber"
he knows = lui sa,
how does he know? = Come sa?
"Come fa a sapere" literally means, "how does he do to know" which is very different from "how does he know?"
You are playing a dangerous game: Spanish and Italian are different, though they have similarities. For example you cannot say conosco a una ragazza as you would say in Spanish conozco a una niña.
Fare a + infinitive is an expression which means "can". Ex. Come fai a mangiare così tanto? "How can you eat so much?" Come fa a ricordarsi di me? "How can he remember (about) me"?
"Does he know me?" literally translates in Italian as fa lui conosce me: it's a structure that one does need to learn by heart. It's pointless to complain that it doesn't make sense in Italian.
Sorry Mutley71... i see you like Grammar a lot. But keep in mind... to argue is to follow the points of view of a discussion. I have never questioned the right translation on this sentence. But the lack of consistency on DL regarding to translations in this Italian course. learn a language it is not only to learn grammar, but the ability to communicate. I hope you agree with me in this. To complain is recommended if we can help to improve the quality of the course. (Where did I complained on the sense of the sentence?) By the way... where in the whole material of the course it's explained that "faire a + infinitive" is the translation of "can"? or are you defending that to learn a language is a game of guessing? I do look for answers and explanations in other webs to solve problems, since DL doesn't offer explanations a all. But that doesn't mean that I can not criticize the quality of the Italian DL course. Obviously with the intention to help the authors to review an improve the course. I enjoy learning languages to communicate with people, not to be an expert on grammar.
I am not saying that " how can he knows..." is not a proper and correct translation. which it is. But the arbitrarity of DL regarding to them. Where, it sometimes demand a literally one, and sometimes not, like in this case. That doesn't help to learn with consistency. Once more, I find mostly these kind of problems in that Italian course, not in the German or French one.
These are the many alternative translations accepted by Duolingo for this very sentence:
How [it is possible to/can he/can she/does she/does he] know [this/that/so] much [on/about/of] [fish/fishes]? How come [he/she] knows so much [on/about/of] [fish/fishes]?
If you have more (correct) suggestions, please state them here and I will gladly add them to the alternatives.