agree with sb ,对象是某人,
agree on sth ,对象是某物,
agree in some field ,对象是某方面,
agree about sth / some field ,对象是某物和某方面.
例：I don't agree on his idea ,but I agree with you .
例：We agree in principle .
例：I don't agree about his idea ,but I agree with you .
例：We agree about principle .