Estoy is the conjugation of the verb estar for the first person singular in the simple present. Here the verb estar is being used as <em>location,</em> describing where I'm temporarily. On the other hand, the simple present tense is used to talk about habitual actions or routines. This tense is also used to describe things that happen now or in the near future. So <em>I am at home </em>an this is as follows:
"Change Fill in the blanks Activity Instructions Change the following sentences by replacing (by replacing) the nouns with the corresponding pronouns. Make any necessary changes."