"There are clouds in the sky."
Translation:하늘에 구름이 꼈어.
My impression is:
끼다 - be covered
구름이 끼다 - be cloudy, be covered with clouds
구름이 꼈어 - is/was (has been and still is) covered with clouds; is/was cloudy.
구름이 껴요 - is covering with clouds (current situation) or is (being) cloudy/clouds over (repetitive; habitual occurence)
구름이 꼈던 - was (had been) covered with clouds (but is no longer the case)
Both are correct depending on what you want to convey to the listener.
,• clouds as clusters of vapour:
하늘에 구름이 있어 - there are (clusters of) clouds in the sky.
• clouds as a misty mass
하늘에 구름이 꼈어 - Lit. The clouds are suspended or the mass of clouds remains suspended in the sky. In other words, there are clouds in the sky.
This expression is best translated into English as "The sky is cloudy" in my opinion.