"We need him."
I understand potřebujeme ho," but I don't understand how potřebujeme jej means the same thing.
It's simple, there are two possible words for the same thing. Check T&N for Pronouns.
"Ho potřebujeme" is wrong but "Jeho potřebujeme" is right? Can someone explain me the difference please?
In Polish you cannot use short pronouns in the begining of the sentence. I believe the same rule applies for Czech, since the languages are closely related.
It is explained in the Tips and Notes: "Some of the forms can only appear in the second position in the clause (like the verb particle se). These "clitics" are the forms listed in italics." Te (with the hat on e) and ho are such forms :)
Yes it's writtern but it is not super clear and the use of italics is just unfortunate.
After reading lots of comments and asking the same questions elsewhere I kind of figured it out: "Ho" and "tě" must follow the second position rule and are more stressed as "jeho" and "tebe". In "potřebujeme ho" we really HIM and in "jeho potřebujeme" we need him but anyone else would be ok too.
"Potřebujeme jeho" is accepted. If that's what you wrote, it should not have been rejected. Perhaps you have encountered the grading bug that turns up now and then... and over which the course team has no control.