What it is The DATIVE case is when the pronoun/noun functions as the INDIRECT OBJECT!
Example Sentances Ich schicke dem Lehrer eine .
WHY? Use dem because because Lehrer is a masculine noun and is functioning as the indirect object. (Hint- you can find an indirect object more easily by putting to/for in front of the noun. If it fits, it is the direct object.)
Example Wem schickst du eine E- Mail?
WHY? Use wem because you are inquiring about the indirect object. You are asking to or for whom.