► "Συγχώρεσέ με" is like "forgive me". You say it after you've done something wrong (or just bumped into someone) and is used to ask for forgiveness.
= "με συγχωρείς;" (you should it as a question : "do you forgive me?")
= "με συγχωρείτε" (plural of "με συγχωρείς" - if you're saying you're sorry to a lot of people)
= "Συγγνώμη" (I'm sorry)
I think that all these could be used when you want to express that you're sorry.
Keep in mind that if you use the first one, "συγχώρεσέ με" it's like you're begging the other person to forgive you. It could sound like you're desperate or kind of like you're giving orders.
► Now, to say "excuse me" in Greek is quite simillar, so you could be easily confused.
When you want to ask someone something, you might want to begin your sentence with "excuse me", just to be polite.
You could do the same with using
"με συγχωρείτε" - plural of "με συγχωρείς" and also, in this case, more polite if you're speaking to a stranger - or
BUT NOT "συγχώρεσέ με". We don't use it like that..
Συγγνώμη, τι ώρα είναι; = Excuse me, what time is it?
★ So, in most cases if you say "συγγνώμη", then you're probably correct, polite and don't have to think of whether you should use plural or not.... And it's the most common one as well!!
Sorry for the long answer~! I hope I helped!! (^o^)/
Ask about anything you don't anderstand and I'll try to help~