"Yo creo que ella está en mi casa."
Translation:I believe that she is at my house.
I'm not that good explaining the grammar rules of spanish, but I'm gonna try to explain the cases when you use the verb to be as "esta" or "es".
"el esta" is for spaces/actions/moods and some others (when you talk or describe something about the subject): El esta en la casa = He is in the house (space), "El esta bien"= He is fine, "El esta feliz"= He is happy.
"El es" is for people or animals (when you talk or describe the subject itself): El es abogado=He is a lawyer, El perro es negro=The dog is black
I hope I explained myself right.
Another difference is more subtle.
El esta aburrido - He is bored (a temporary condition)
El es aburrido - He is boring (a personal trait and less temporary condition).
So be careful with estar and ser! It takes a little time to get used to the differences for native English speakers because we don't have two forms for the verb 'to be'.
vimi- It's true for doubts that you use subjonctive, but not doubts between 2 verbs, but doubts for some situations. If you say, I don't believe he will come, we use subjonctive in place of future like in English. no creo que venga., because it's a doubt, you don't know for sure if he will come or not. The doubt can't be about 2 verbs that you don't know which one to choose.