When the direct object is a specific person (someone you can picture in your mind), you MUST use the preposition “a”.
I see Manuel.
I see the girl.
Veo a Manuel.
Veo a la chica.
When the direct object is a thing NEVER use the preposition “a”.
I see the car.
Veo el coche.
“A” must be used before each person if there is more than one.
I see Manuel and Suzi.
Veo a Manuel y a Suzi.
“A” must also be used for specific pets which you would let into your house, but not before pets in general.
I love my dog.
I love dogs.
Amo a mi perro.
Amo los perros.