There are several numbers that drop their lower consonant when in use. 둘 셋 넷 all become 두 세 네
This is correct. These numbers all change form when a counter is added. Otherwise they are hard to pronounce. For the same reason 하나 becomes 한. 둘 - 두 셋 - 세 넷 - 네 다섯 onwards remain the same.
It says "들다" means "holding" but says "The smart student is holding four books." is incorrect. I don't understand.
I missed carrying...had no clue to that. I just said that the smart student was with four books. :(
I used, "The smart student is holding four books," the response was accepted as correct. The issue is with "bright". Duolingo was looking for a dictionary definition of smart--bright would be considered colloquial but understood by those whose first language is English.