Each turn consists of 4 phases, one for each action in a turn. If an effect grants you additional actions per turn (such as Hasted), then you get to act in additional phases as appropriate. Each character gets to take a single action during each phase.
Multi-Action Activities
When performing an Activity that takes more than 1 Action (such as casting a 2 Action spell), you perform the two actions separately. Usually, the Activity makes it apparent what each Action represents (for example, when casting a spell, the first action might be the Somatic component and the second action might be the Verbal component).
Regardless, the Activity takes effect when all the actions for it have been spent.
Activities can (often) be broken up with movement or other non-concentration-breaking actions. For example, while casting a 3 action spell, you could cast the Somatic component in phase 1, then move in phase 2, then cast the Verbal component in phase 3, then the Material component in phase 4, completing the spell.
Attacks and other vigorous actions require you to make a Spell Attack Roll with a -2 Circumstance Penalty with DC equal to your Spell's DC in order to maintain concentration on your spell. If you fail, the spell is unable to cast, or potentially backfires. Moving, even running, will rarely force you to maintain concentration in this way, but falling, being shoved, sneaking, or balancing usually require you to concentrate.
