The personal a

The personal a
 

I am working my way through a textbook and it is great so far, but I have come to a snag with the personal a.
The author gives a brief explanation of when to use it but then further on in the book she seems to exclude it. More than likely I just don't understand it fully. I'll give a couple of examples.
Why don't these use the personal a? (I realize that I'm missing some accents but don't know how to insert them)

?Que compro Marta esta manana?
?Acepto Maria la invitacion?
?Tomo Maria la cena en su casa?

Why are these not as follows?

?Que compro a Marta esta manana?
?Acepto a Maria la invitacion?
?Tomo a Maria la cena en su casa?

Thanks. I'm looking forward to figuring this out.

The personal 'a' precedes a direct object. The women above are all subjects.

There's an 'Accents' drop-down menu just above where you type that you can use to insert the special characters.

Thanks, Rusty