que / ce que are used when they are the object complement in the relative clause as well as when they stand on behalf of inanimate (Je ne sais pas ce que le garon a fait. Je connais un secret que je ne rvlerai jamais)
qui/ ce qui are used when they are the subject in the relative clause (Je ne sais pas ce qui se passe; j’ai repris du gateau qui est si bon). Qui can also be used on behalf of persons when it is an object complement in the relative clause (je ne sais plus qui je dois croire).
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“ce que” is used when a NOUN follows it
“ce qui” is used when a VERB follows it
for example:
“Je ne sais pas ce que le garon a fait” (le garon being a noun)
“Je ne sais pas ce qui se passe” (se passer being a verb)
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