1. With verbs of movement like ir(to go)
YO VOY A LA IGLESIA=I go to the church
2. When the direct object of a verb is a person or animal, not a thing.
PERSON/ VISITO A MIS ABUELOS=I visit my grandparents
THING/VISITO EL MUSEO(no a)= I visit the museum
3. To indicate the indirect object/to whom something is being done
YO LE DI UN REGALO A DAVID=I gave a present to david
4. To indicate how something is cooked or done
POLLO AL HORNO=chicken cooked in an oven
POLLO A LA PARILLA= chicken cooked on a grill