I do not think so. Sapere means "to posses an information" while conosciere means "to be familiar with". The same concept (two different words for to know) is used in many other languages including my native. For example, I can know (conosciere) the rules of a game, which allows me to know (sapere) why the referee blows the whistle.
In the present, conoscere can mean either "to know someone" or "to meet someone" BUT in the past tense the meanings are not so flexible. If you use "io ho conosciuto" it means "i met someone". This tense is a particular moment in time and can't be translated as "i knew someone." To say "i knew someone" is implying that you knew them over a period of time. I order to say "i knew someone", you would use a different past tense of the verb. "io conoscevo".
http://italingua.ning.com/profiles/blogs/so-ho-saputo-sapevo-che-confusione (this page also has sapere, which is similar, but if you scroll down it talks about conoscere)
IMHO it is correct. The Present Perfect is usually used in this sentence because it is implied that the fact of "not knowing" persist until today. However, in some cases, the period of "having the possibility of knowing him, but not doing it" is completely in the past (for instance, if he is dead). In such a case, the Simple Past would be used.
Thank you again. And I got through that level. So very happy now! I think I see what you mean by the object also being the subject of essere ... but is a good rule of thumb that avere verbs change according to the object, verbs with essere change according to the subject? 4 languages - wow! I'm finding one enough of a handful!
i thought i sent the reply, but now i can't see it... so i hope there won't be a duplication. so verbs that work with "essere" are the verbs that express moving or existing. so they don't have an object. and those verbs that have an object use "avere". so "essere" verbs like essere, vivere, andare.... etc change according to the subject. all the other verbs change accordig to the object.
Got it - I think! Thank you once more. Another general question - for anyone really - why can't I identify myself as British rather than American on this site? Not that I would have anything against being American, if I was! Maybe a question for the Duolingo team, rather than the users, though if anyone knows the answer...
From first report, can take up to 4 months, or so I hear. They get a LOT of suggested changes, and (hopefully) check all those suggestions before they make corrections. Some of the longer discussion topics seem to end with a rash of "yay! finally!" - and those I have seen seem to span about a 4 month mark as well.