"Friends are good."
Here is the issue: I selected: 친구는 좋습니다 and got it right. The help button shows: 친구들은 좋습니다 as the right answer. There is no option to select 친구들은.
They're both correct. "A friend is good" is the same as "friends are good". 들 only clarifies plurality
Why is the answer in singular (친구) but the question is un plural (friends) form?
I don't know what you mean but I'll try to help. 친구 Is friend and attaching 들은to the end makes it plural: friends.
Exactly. The prompt is "friends" therefore you should pick 친구들. But that's wrong.
Koreans normally don't normally clarify between singular and plural, except when needed for emphasis. For instance, in Korean "I like apples." is normally written as "사과 좋아해." instead of "사과들이 좋아해."
What's the different between 좋습니다 (good), 착합니다 (nice), and 친절합니다 (kind)? Are they interchangeable for a sentence like: "The woman is good/kind/nice"?
This is a general statement about friends. Also the verb 좋다 can mean "to like" if a topic is specified or implicitly a person. E.g. (나는) 사과가 좋습니다 reads "With respect to me, apples are good" which basically means "I like apples". Note how 사과가 좋다 differs from 사과를 좋아하다 ("apples are good" vs "to like apples")
I read somewhere that in korean you shouldn't use 를 after objects and use 가/이 instead, Is it correct to use both here?
The advice of that you "don't use 를/을 after an object" makes no sense, since 를/을 is the object marker.
And if you changed the 은 in this sentence to 이, the meaning would change to more like "My friends are good."