If "melihat" is translated as present tense: "He sees that you are changing" or "He sees that you have changed." If "melihat" is translated as past tense: "He saw you changing" or "He saw that you changed" or "He saw that you had changed." (In any version, the change must occur before any aspect of it can be observed.)
It is a valid sentence, if awkward in isolation.
"He was very proud of you. He saw that you change. Every time you encounter a new challenge, you try something new to overcome it."
"Dia melihat bahwa kamu berubah."
Translation:He saw that you change.
More translations should be accepted :
He saw that you changed.
He saw that you have changed.
He saw that you had changed.
He sees that you have changed.
I tried the above, and they were all rejected.
At least this is a lot better than the Vietnamese course - in spoken Vietnamese you can generally omit the tense markers, just like in Indonesian - but that course adheres to a strict one-to-one policy of translating sentences, which can catch you off-guard if you are used to using day-to-day Vietnamese.
I think that maybe the English should be 'He saw that you have changed'. It sort of makes sense as is, but maybe that would be better.
Yang gives more information about a noun, whereas bahwa introduces an action (I think the term is subordinate clause?)
So in this case "Kamu berubah" could be a complete sentence, but we put "bahwa" in front of it to make it part of a longer sentence.