I am sorry but

I am sorry but, this phrase use when we refuse invitation from our friends, it's simple look this patern

1.もうしわけありませんけど。。。- mousiwake arimasenkedo
2.すみませんけど。。。 - sumimasen kedo
3.ごめんなさいけど。。。- gomennasai kedo
I am sorry but　。。

those third phrase have same meaning,number one more polite than number two and three, see this example below :

1.すみませんけど ほうしありません。- sumimasen kedo housiku arimasen, - i'm sorry but i'd rather you didn't
２．すみませんけど、ちょっと忙しいので。sumimasenkedo, chotto isogashinode. - i'm sorry but i'm little busy.
３．ごめんなさい、じしんがないですね。gomenasai kedo,jishin ga naidesune. - i'm sorry but i don't have feeling to do that

it' easy right ! take your note and make your own sentence, have fun !