Because of the auxiliary "can" "He has two apples" becomes "He can have two apples". "He wakes up early" becomes He can wake up early" Words after an auxiliary or modal verb are always expressed in the infinitive.

"It couldn't have been easy for her." would be correct in this sentence. Because "could" is past tense, and "can" is present tense. It is difficult for me to explain the difference in "has" and "have", but I believe this website may be helpful for you: http://www.differencebetween.net/language/difference-between-has-and-have/

can/can't + have Simple!

Gracias Atarilynx!

Es la diferencia entre: No puede ser facil para ella o No puede haber sido facil para ella y no puede es facil para ella o No puede hay sido facil para ella Como se escribio antes, have es el infinitivo And actually, I think it might be best to say: It couldn't have been easy for her Since the phrase seems to refer to something in the past