He is the first dentist to work here.
a. es el primero dentista trabajar aquí.
b. es el primer dentista en trabajar aquí.
c. es el primero dentisto trabajar aquí.
d. es el dentista primero trabajar aquí.

ah, so ustedes and nosotros are my 'go-to' if i am using spanish mainly in the states? interesting, but still slightly confusing, haha."

it seems that you are a little confused about subject pronouns. yo=i, tú/usted=you, nosotros=we, ustedes=you(plural), ellos/ellas=they.

in the case of vosotros, it is an informal replacement for "ustedes" in spain. similar to using "you all" in english.

even though it is not generally used in conversation outside of spain, it is used many times in literature. i would suggest having at least some familiarity to that form, at least to recognize what it is.

ti

explanation:

the answer is ti because it means you