Learn Portuguese (Portugal): A minha mãe chama-me para o jantar.

me is a direct object pronoun meaning "me". In European Portuguese, these short object pronouns usually attach to the end of the verb with a hyphen in basic, affirmative sentences. So instead of putting it before the verb, "calls me" becomes chama-me.

If I wanted to say "My mother calls him" instead, how would I change chama-me?

You would replace me with the direct object pronoun for "him", which is o. The verb phrase would become chama-o.

Why is there an A at the very beginning of the sentence? Doesn't minha mãe already mean "my mother"?

In European Portuguese, it is a standard rule to put a definite article (o, a, os, as — meaning "the") right before a possessive word. Because mãe is a feminine noun, we use A. It literally translates to "The my mother", which is exactly how people naturally speak in Portugal!

Can jantar also be a verb? Why is there an o before it here?