진하다 : composed of or containing objects, particles, etc., close together; dense, dark, concentrated (used for color, concentration of liquid,smell)
연하다 : composed of or containing objects, particles, etc., widely separated; sparse:
thin vegetation. soft, weak (used for color, concentration of liquid, smell)
e.g.
그녀는 언제나 진한 향수 냄새를 풍긴다
그녀는 연한 초록색 옷을 입었다
두껍다 : having relatively great extent from one surface or side to the opposite (used for material)
얇다 : having relatively little extent from one surface or side to the opposite (used for material)
e.g.
그 얼음은 당신의 무게를 지탱할 만큼 두껍다
그녀는 얇은 옷 하나만을 걸치고 있다