Hi Wind. No, you can't use the two interchangeably and your friend is correct. Let me explain. "I agree" means that you do agree with whatever is being talked about. Eg. "I agree with you that we need to increase our sales" = "Yes, we do need to increase our sales". "I'd agree" = "I would agree" which means that you WOULD agree if a CERTAIN CONDITION WERE MET. eg. "I'd agree with you that we should increase sales but I think that cutting our production costs is more important". = "I WOULD agree that we should increase sales, BUT [I DO NOT AGREE] because I think we should cut our costs instead". "I'd agree with you on that but I think you're wrong about a couple of things". = "I WOULD agree with you on that BUT [I DO NOT AGREE] because I think you are wrong about several things". I hope this helps! If not just post a comment and I will try to elaborate. Take care and good luck!