He can't be at his grandmother's. = He couldn't be at his grandmother's.
He can't have been at his grandmother's yesterday. = He couldn't have been at his grandmother's yesterday.
Znamenají výše uvedené věty to samé (za předpokladu, že jsou správně)? = Určitě není u babičky. / Určitě včera nebyl u babičky.
Děkuji. =)