Learn Chinese Grammar: 但是(dànshì=but) vs. 可是(kěshì=but) vs. 不过(bùguò= but):

不过 is more informal and mostly used in spoken Chinese. 不过has a softest intensity of tone (compared with 但是 and 可是). 不过 is often associated with the positive result. It includes, is used to say 'no' or 'negative thing' in a softer way (to avoid hurt someone's feelings) so it sounds 'positive.' (Here it also depends on personal preference).

Examples:

The house is new, but it's very dirty.

a.房子(fángzi=house)很(hěn=very)新(xīn=new)，但是(dànshì=but)很(hěn=very)脏(zāng=dirty) 。

b.房子(fángzi=house)很(hěn=very)新(xīn=new)，可是(kěshì=but)很(hěn=very)脏(zāng=dirty) 。(More used for negative result)

c. 房子(fángzi=house)很(hěn=very)新(xīn=new)，不过(bù​guò)很(hěn=very)脏(zāng=dirty) 。(Make negative result sounds 'positive')