When you start to get more 'advanced' in Greek grammatical studies, you really begin to discover that nothing is what you thought it was. Here are three examples.
Traditional grammars tell you that Greek has an active, middle, and passive voice. And they parse out the meaning like this: "I kill you" (Active); "I am killed by you" (Passive); "I kill you with some relationship to myself" (Middle). And then they tell you that there are deponent verbs which 'lost' their active voice and just use middle and passive forms for active and passive.
But most in this field now agree that this is not really correct. In fact, we would be better off speaking of two voices: active and 'subject reflexive'. The old 'middle' and 'passive' are really subdivisions of a form of verb that sees the subject as somehow involved in the action, hence 'subject reflexive'. And the so-called 'deponents' are not deponents at all. Their 'core meaning', if we can indulge that for a moment, already involves the idea of "subject affectedness", which is why they aren't unusual, they never have an "active" or "common" voice to begin with.
Traditional grammars tended to teach the Greek perfect tense-form along the lines of the Latin perfect. That is a shame because it was mistaken. More recent work has highlighted the complexity and interrelationship of tense and aspect in Greek, particularly the way aspect is primary outside the indicative mood. So now there is considerable debate about what aspect the perfect tense has. If you go with Con Campbell, the perfect tense-form is actually imperfective in aspect, and marked for 'proximity'. Porter et alii describe the perfect as 'stative' in aspect, a more widely held view. Some still hold that the Perfect is perfective, but I would describe this as a relatively minor view. Overall, the momentum of opinion is that the perfect is not perfect!
Absolutes are not absolute
This is the point that really inspired me to write this post. If you've studied Greek, you'll know that the genitive absolute is a construction that involves generally a noun and a participle, in the genitive case, usually at the start of a sentence, that tells you something correlated to the sentence, but having no grammatical connection to the rest of the sentence.
Galatians 3:25 is a good example: ἐλθούσης δὲ τῆς πίστεως οὐκέτι ὑπὸ παιδαγωγόν ἐσμεν. trans. "with the coming of Faith, we are no longer under a paidagogos."
The key element of it being 'absolute' is that 'faith' is not attached to any other grammatical feature of the rest of the sentence. If it were, so traditional theory goes, it would be in the appropriate case and so would the participle.
This is all well and good, until you realise that many, many times the relevant noun in the genitive absolute phrase does appear in the rest of the sentence. So, for example, Mark 5:2:
καὶ ἐξελθόντος αὐτοῦ ἐκ τοῦ πλοίου εὐθὺς ὑπήντησεν αὐτῷ ἐκ τῶν μνημείων ἄνθρωπος ἐν πνεύματι ἀκαθάρτῳ,
Trans. "as he got out of the boat, immediately a man from the tombs, unclean in spirit, encountered him".
The αὐτοῦ of the genitive absolute is Jesus, and this is the same referent as the αὐτῷ later in the sentence.
I don't have time to fully explore this, but instead I refer you to an excellent article Lois Fuller, "The 'Genitive Absolute' in New Testament/Hellenistic Greek: A Proposal for clearer understanding." Fuller shows how 'absoluteness' is not in fact one of the essential properties of this construction - the absolute is not absolute at all.
Things are just not as simple as they seem. Grammatical research continues to sharpen our knowledge of Ancient Greek. One of the things this should do, though, is change how we teach introductory Greek. This isn't 'advanced' knowledge, and one shouldn't have to unlearn 1st year Greek in order to do 3rd year Greek.