I stand corrected. You are right, I totally confused it with πώς/πως, and assumed it worked the same. It should not have an accent.
I'd better delete my comment then, as it is I see now misleading, please confirm that you agree before I do.
Πεφτω απο τα συννεφα, και ομολογω οτι την πατησα εδω. Πιθανως να το ειδα σε πολυτονικα κειμενα, ή απο καποιον που το τονιζει, και να θεωρησα οτι ειναι σωστο. Τωρα που το κοιταξα τελικα, δεν τονιζεται.
Τι is always written without an οξεία, but I understand why you would be confused by this.
Πώς and πού are written with the οξεία as interrogatives but not as conjunctions. This is because they can have different meanings as conjunctions, depending on whether you include the οξεία. For example:
Πρέπει να σου πω πως συνέβηκε. (I must tell you that it happened.)
Πρέπει να σου πω πού συνέβηκε. (I must tell you where it happened.)
That's probably incorrect Greek, but I hope it demonstrates my point.
EDIT: teopap mentioned that I got πώς and πού mixed up. See his comment for more.
Your examples might be more clear if you use πώς in the second sentence, as you had it before:
- Πρέπει να σου πω πως συνέβη. (I must tell you that it happened.)
- Πρέπει να σου πω πώς συνέβη. (I must tell you how it happened.)
(And that is how you use συνέβη = "it happened", i.e. exactly as you used συνέβηκε. I am just not sure if συνέβηκε is a valid word, but συνέβη surely is.)
"What is important ABOUT him" was marked wrong. Glosbe gives this example: "And like so many secrets, it was the most important thing of all about him." for "Και όπως τόσα πολλά μυστικά... ήταν το πιο σημαντικό πράγμα απ'όλα γι'αυτόν." Shouldn't my "about" be okay here?