"He saw that you changed."
Translation:Dia melihat bahwa kamu berubah.
'berubah' (instead of 'merubah') should be used here.
BTW, the new spelling is 'mengubah' (instead of merubah).
"Kamu berubah" = "You changed."
"Kamu mengubah sesuatu." = "You changed something."
"mengubah" needs an object, an object that is being changed.
"mengubah" is a transitive verb, it has an object in the sentence.
"berubah" doesn't have an object in the sentence.
"berubah" is an intransitive verb (it has no object).
The thing that is changing is the subject in the sentence.
Here is a topic where I tried to explain the difference: