YOU SAID:
Markus is not the brightest person
INTO JAPANESE
マーカスは、優秀な人ではないです。
BACK INTO ENGLISH
Marcus is not excellent.
INTO JAPANESE
マーカスが良くありません。
BACK INTO ENGLISH
Marcus is not good.
INTO JAPANESE
マーカスは良くない。
BACK INTO ENGLISH
Marcus is not good.
You've done this before, haven't you.