so it's 2 sentance incorrectly joined
exg correctly joined
John is having a birthday party, but you can't go.
incorrectly joined
John is having a birthday party, you can't go
sometimes the 2 sentances should be written as seperate sentances and seperated with a period
here's an easy way to remember a run on sentance is like 2 trains that run into eachother, it doesn't have the right connecter