In Spanish, when the direct object is a person, it is preceded by the preposition "a." This word has no English translation.
Pedro llama a María.
Pedro calls María.
From the point of view of the English speaker, the personal "a" appears to be an extra word. From the perspective of the Spanish speaker, the personal "a" is required, and to not use it is a serious error.
Pedro llama a María.
The personal "a" can also be used if the direct object is a domesticated animal, especially a pet, provided that the speaker attaches some sort of personal feelings towards the animal.
La mujer acaricia a su perro.
The woman pets her dog.
El perro persigue a la gata.
The dog chases the cat.
The personal "a" isn't used when the direct object is not a person or is an animal for which no personal feelings are felt.
Bebo la leche.
I drink the milk. -- milk is neither a person nor an animal
Miro la jirafa.
I look at the giraffe. -- no personal feelings are felt towards the giraffe
The personal "a" is not used after the verb Tener, or the verb form hay. This is true even if the direct object is a person.
Tengo dos hermanos.
I have two brothers.
Hay cinco chicas.
There are five girls.
If the direct object is an indefinite person, the personal "a" is not used.
Necesito médico.
I need (any) doctor. (or)
I need medical assistance.
Necesito jardinero.
I need (any) gardener. (or)
I need someone to tend my garden.
Because this Spanish grammatical structure has no English equivalent, it is normal hat the student will forget to use it until a pattern of use has been established in his or her brain. Remember, to not use the personal "a" is a serious error, and the student should try to remember to use it when appropriate.
http://www.studyspanish.com/lessons/persa.htm