Hmmm Optatif is out
Aoriste is in
Dative (datif?) is out too. Dual number also. Oh there are so many things!
http://www.in.gr/dictionary/ is a good dictionary but is in modern Greek.
Dictionaries:
http://www.komvos.edu.gr/dictionaries/dictonline/DictOnLineKri.htm - Greek-Greek dictionary
Grammar:
http://modern-greek-verbs.tripod.com/contents.html - a huge alphabetical list of conjugated Greek verbs
http://www.cogsci.indiana.edu/farg/harry/lan/grkgram.htm- a brief overview
http://www.cogsci.indiana.edu/farg/harry/lan/grkmore1.htm - syntax
http://www.greece.org/gr-lessons/gr-english/grammar+syntax.html - overview of grammar and syntax
http://www.tauntaun13.150m.com/Lessons.htm - Basic Greek Lessons
http://www.geocities.com/alexandrosworld/Greek/Home.html - Phonology, Morphology, word order, a brief history
http://www.media.uoa.gr/language/ -Τα Ελληνικά ως ΞÎνη Γλώσσα
http://babel.lexilogos.com/ellenika/ - Grammaire de Grec Moderne
http://www.geocities.com/hmpwgreek/irregular-verbs.pdf - Irregular verbs
copy/pasted by the resources from another forum where I moderate
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(I know, I'm bad)
the cosgi indiana link is the one you will find more interesting I guess.
Check it (and the rest if you want out) and then I can go into details of how modern Greek differs from ancient Greek.
I think the babel.lexilogos one will help you too since it is in French (haven't browsed it thoroughly though)
In general, the tenses (temps) remain the same though the endings and a few other things have changed.
If you know ancient Greek you can read Katharevousa (google the term) quite easily. Modern Greek will need some ... "adjustment"
Perhaps the greatest change is pronunciation though.