Why can't we use είπα here?
Because λέω is to say something or to tell someone something — not to speak or talk (= μιλάω).
I'm confused as to why την μίλησα didn't work here? thanks :)
I'm confused as to why την μίλησα didn't work here?
Because you didn't "speak her" -- you "spoke TO her".
Indirect object, so you use the genitive της instead of accusative την.
But in a different sentence it was something like "Το παιδί δε με μίλησε". This is accusative case, so why in this case genitive is needed?
I don't remember having such a mistake around - it must be "...δε(ν) μου μίλησε".
I saw: Το παιδί δεν μου μίλησε. Genitive case.
"Μίλησα να της" is incorrect?
Yes , it's incorrect. Talk to someone = μιλάω σε κάποιον. So, to is translated as σε, not να.
I talked to her - (Εγω) μίλησα σε αυτήν/ (Εγώ) της μίλησα. ^.^
Oh. Thank you very much. My dictionary gave me: Μιλώ με κάποιον για κάτι. I was in doubt translating με as TO. Now, Μιλώ ΣΕ κάποιον για κάτι, no doubt. Thanks again.
Μιλάω με κάποιον and Μιλάω σε κάποιον have the same difference as talk with someone and talk to someone in English.
would "μίλησα της" be correct?
No, it would not.
Short-form pronouns such as της come before the verb, not after it. (Except in commands.)