Can IG-88 attack twice or not? Also why can he or can't he?

Can Vader use brutality? Why can he or can't he?

Can Finn segnis use the assault ability to give multiple attacks to troopers? Why or why not?

Yes.

Yes, but he cannot attack again.

Yes

cards always override the rules.

Imperial Assault is an exception-based game. That is, the basic rules always apply, unless something on a character's card or other game piece states otherwise. Your questions are all centered around the rule that no character can attack more than once on its turn; that rule is true and applies to just about all characters, but there are exceptions, and all of those exceptions are explained on the Deployment cards of those characters. All special abilities (such as Brutality or Assault or anything else) have the ability to override the basic rules on those points. Therefore, in answer to your questions:

IG-88 can attack twice, because he has the Assault ability.

Vader can use Brutality, which requires only 1 action. When he uses Brutality, he makes attacks against 2 separate enemies. He cannot use Brutality twice, because it is a special ability, and no character can use a special ability twice on its turn.

Fenn Signis doesn't give the troopers Assault because he has it (it only affects him), but rather because his Trooper Assault ability gives them an extra attack.

OK Thanks For The Explanations