Japanese phrase
___があるので、行けません。 ___ ga a ru no de i ke ma se n
Meaning
I can't go because I have ___.
When to use it
A prior plan prevents the speaker from going. The wording politely gives a general reason for declining an invitation and does not sound overly formal.
Example dialogue
今夜、食事に行きませんか？ kon ya shoku ji ni i ki ma se n ka
Would you like to go out for dinner tonight?
すみません、予定があるので、行けません。 su mi ma se n yo tei ga a ru no de i ke ma se n
Sorry, I can't go because I have plans.