다고

reports a said or thought description

Packages a statement so a following verb can report it — 많다고 들었어요 is "I heard there are a lot", 지루하다고 생각해? is "do you find it boring?", 챙겼다고 생각했는데 is "I thought I'd packed it". This is the shape used when the reported clause is a description, a past event, or 있다/없다. When the reported clause is a plain present action you get -ㄴ다고 instead (마신다고), and when it is a bare noun you get -라고. The verb that follows tells you the source: 하다 for said, 듣다 for heard, 생각하다 for thought.