"My mom is calling me."
Translation:אמא שלי קוראת לי.
Because certain verbs require certain prepositions. English pronoun "me" can be translated in many ways into Hebrew, לי and אותי being only two of several. After the verb קורא, if you want it to mean "call", you need to use לי - "calls me/is calling me". If you used את after this verb, it would imply "reading", which is another meaning of the verb. So, אמא שלי קוראת אותי would awkwardly translate to "my mom is reading me" as in I am the thing she is reading, not that she is reading to me.