Well, disagreeing with someone is okay as long as you back your argument up with reason or facts instead of just saying "I don't think so". You could say "I don't agree with you because" and then back it up with valid counter arguments however I think it's even more polite to skip the "I don't think so" part and instead show your disagreement in the form of a validated counterargument without having to directly say that you disagree with them. Ofcourse the most polite way would be to not say anything at all, however then your opinion wouldn't be heard obviously which could cause other problems in the long run so I wouldn't recommend that approach.
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Well, disagreeing with someone is okay as long as you back your argument up with reason or facts instead of just saying "I don't think so". You could say "I don't agree with you because" and then back it up with valid counter arguments however I think it's even more polite to skip the "I don't think so" part and instead show your disagreement in the form of a validated counterargument without having to directly say that you disagree with them. Ofcourse the most polite way would be to not say anything at all, however then your opinion wouldn't be heard obviously which could cause other problems in the long run so I wouldn't recommend that approach.