Is Spanish the same in all Spanish-speaking countries?
No, there are some grammar or word variations between different Spanish-speaking nations. For instance, Spain uses vos/vosotros for the English word "you", while Mexico does not.
How do you conjugate verbs into the passato prossimo verb tense?
First, you need the auxiliary, which is just avere or essere conjugated in the present tense. If the verb involves a change of state/change of location verb, like muovere or crescere, then the past participle is essere. If it is any other verb, then you use avere. Once you conjugate the auxiliary, afterwards you need to conjugate the past participle of the verb. Typically, for -ARE verbs the past participle is -ATO, for -ERE verbs it is -UTO, and for -IRE verbs it is -ITO. However, there are a few irregular past participles, such as preso instead of prenduto for prendere. So, if you want to say "I ate a banana", you would say "Ho mangiato una banana".
Why are facial expressions especially important for you to be understood in American Sign Language?
Because unlike any spoken language, there are no inflections of the voice in ASL, and therefore facial expressions are used in place of that. They are also used for grammar purposes, as well, like when asking questions, one either raises his or her eyebrows for yes/no questions, or lowers them for who, what, where, why, and how questions. They essentially allow people to understand each other perfectly when only communicating with hands and body movements.