A

A is used before a direct object if it is a person. Direct objects following the verb tener form an exception.

SpanishEnglish
conozco a JuanI know Juan
ayudar a alguiento help someone
no oigo a nadieI do not hear anyone
tengo una hijaI have a daughter

Some frequently used phrases with a:

SpanishEnglish
a lo másat most
a propósitoby the way
al principioat first