Pathfinder has many specific rules, but you'll also want to keep these general guidelines in mind when playing.

The Gm Has The Final Say

If you're ever uncertain how to apply a rule, the GM decides. Of course, Pathfinder is a game, so when adjudicating the rules, the GM is encouraged to listen to everyone's point of view and make a decision that is both fair and fun.

Specific Overrides General

A core principle of Pathfinder is that specific rules override general ones. If two rules conflict, the more specific one takes precedence. If there's still ambiguity, the GM determines which rule to use. For example, the rules state that when attacking a concealed creature, you must attempt a DC 5 flat check to determine if you hit. Flat checks don't benefit from modifiers, bonuses, or penalties, but an ability that's specifically designed to overcome concealment might override and alter this. While some special rules may also state the normal rules to provide context, you should always default to the general rules presented in this chapter, even if effects don't specifically say to.

Rounding

You may need to calculate a fraction of a value, like halving damage. Always round down unless otherwise specified. For example, if a spell deals 7 damage and a creature takes half damage from it, that creature takes 3 damage.

Multiplying

When more than one effect would multiply the same number, don't multiply more than once. Instead, combine all the multipliers into a single multiplier, with each multiple after the first adding 1 less than its value. For instance, if one ability doubled the duration of one of your spells and another one doubled the duration of the same spell, you would triple the duration, not quadruple it.

Duplicate Effects

When you're affected by the same thing multiple times, only one instance applies, using the higher level or rank of the effects, or the newer effect if the two are equal. For example, if you were using mystic armor and then cast it again, you'd still benefit from only one casting of that spell. Casting a spell again on the same target might get you a better duration or effect if it were cast at a higher rank the second time, but otherwise doing so gives you no advantage.

Ambiguous Rules

Sometimes a rule could be interpreted multiple ways. If one version is too good to be true, it probably is. If a rule seems to have wording with problematic repercussions or doesn't work as intended, work with your group to find a good solution, rather than just playing with the rule as printed.