Some degree of mixing is unavoidable. My answer will cover multiple aspects. The first comes today.
As a foreigner, to speak modern Greek, even on the level of a seventh grade student, a cursory knowledge of "ancient" grammar is indispensable. This is not due to the many Idioms, which you might view as dispensable, but rather to a slight incompleteness in the grammar of the δημοτική, the vernacular we now speak. I don't say this to belittle the δημοτική which I love and grew up with. It is completely natural in a society where the supposed intelligentsia spoke a different idiom for much of the twentieth century. China therefore experiences the same challenges with its relatively new common idiom!
Let me give an example. You want to translate the sentence:
Last year, our company exported one tonne of tangerines.
As a dutiful student, you know "export" is εξάγω. But how to form the aorist? The native speakers will tell you the aorist is εξήγαγα and the παρατατικός εξήγα, but these forms will make little sense to you, until you learn about χρονική αύξηση in Ancient Greek. Don't get me wrong! You will still have to memorise like hell, but your ears will be tuned and it will not be a problem! :) So:
Πέρυσι η εταιρία μας εξήγαγε έναν τόνο μανταρίνια.
Μεχρι πέρυσι η εταιρία μας εξήγε μανταρίνια. Από φέτος εξάγει πορτοκάλια!
Until last year our company used to export tangerines. From this year on, it exports oranges!