Facing a raise: 3-bet or fold
An open in front of you changes everything — most hands you'd happily open yourself are now folds. The skill is reading who raised.
The baseline (full reasoning in the 3-bet guide): versus a tight early open — 3-bet QQ+/AK, call medium pairs and the best suited hands, fold the rest (yes, even AQo). Versus a wide late open — 3-bet TT+/AQ+, call small pairs and suited broadways. R marks the raiser.