Best Answer - Chosen by the Asker
(1) αδύνατος
there's no άδυνατος in Greek.
The correct spelling (stress position) is 'αδύνατος'. It both means 'impossible' and 'thin' for a person. It can also mean 'weak' but for skill or knowledge of sth, not in strength.
Examples:
Αυτό είναι αδύνατο (=this is impossible)
Αυτός είναι πολύ αδύνατος (=he is very thin/he's skinny)
Είμαι αδύνατος στα μαθηματικά (= I'm not good at maths / I'm weak at maths)
I don't know how to leave you a voice response!! :P check out this TTS software though:
http://www.oddcast.com/home/demos/tts/tts_example.php?sitepal
select Greek and type αδύνατος. The pronunciation you'll hear is correct and natural.
(2) γνωστός-διάσημος
They both mean 'famous' and they can both be used for people or places, although maybe διάσημος is used for people more often.
Actually διάσημος means 'famous', whereas γνωστός means 'known' or 'well-known'
Ένας διάσημος / γνωστός ηθοποιός ==> a famous actor
Η Σάμος είναι γνωστή για τα κρασιά της ==> Samos is known for its wine
