Verified answer

I just saw your father on the street

I've just seen your dad on the street.

Eso sería algo como "he visto (o acabo de ver) a tu papá en la calle"

I just saw. (Acabo de ver.)

I just saw your father in the street (Acabo de ver a tu papá en la calle.)

I have just seen your father in the street / I've just seen your father in the street

just es un adverbio que acompaña al present perfect tense

por lo tanto la oración hay que construirla usando ese tiempo de verbo

Los adverbios que identifican al Present perfect tense son :

already / since / for / just / yet

I just saw your father ON the street = al lado de la calle

I just saw your father IN the street = en el medio de la calle.

Es complicado

Juan david Gonzales is right!!!

I just saw your father ON the street!!