HasteHaste is a static ability.
If a creature has haste, it can attack even if it hasn’t been controlled by its controller continuously since their most recent turn began. (See rule 302.6.)
If a creature has haste, its controller can activate its activated abilities whose cost includes the tap symbol or the untap symbol even if that creature hasn’t been controlled by that player continuously since their most recent turn began. (See rule 302.6.)
Multiple instances of haste on the same creature are redundant. (CR 702.10)
BoastBoast is a keyword that adds additional rules to the activated ability that follows it. “Boast — [Cost]: [Effect]” means “[Cost]: [Effect]. Activate only if this creature attacked this turn and only once each turn.”
Effects may refer to boast abilities. If an effect refers to a creature boasting, it means its boast ability being activated. (CR 702.142) —
: You may change Arni's base power to 1 plus the greatest power among other creatures you control until end of turn. (
ActivateTo activate an activated ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect. Only an object’s controller (or its owner, if it doesn’t have a controller) can activate its activated ability unless the object specifically says otherwise. A player may activate an ability if they have priority. See rule 602, “Activating Activated Abilities.” (CR 701.2) only if this creature attacked this turn and only once each turn.)