- I went to the store I bought milk.
- I went to store to buy milk and, while I was there, I met my friend George who was buying white bread, so I stopped to talk to him, and I pointed out that whole wheat bread is a much healthier choice—he didn't care though because he likes the taste of white bread.
If you chose number 1, you get a plate of virtual cookies.
I believe that most people would choose number 2, because when they hear "run on" they think of a sentence that goes on and on; therefore, a long sentence. However, that is not the definition of "run on sentence." A run on sentence is "a written sequence of two or more main clauses that are not separated by a period or semicolon or joined by a conjunction." (Dictionary.com) In other words, two sentences jammed together without anything to join them properly into one sentence.
There are actually two kinds of run on sentences: fused sentences and comma splices. In the example above, number 1 is a fused sentence. I don't see these that often, but I do see a lot of comma splices, even in published works. The Song of Ice and Fire books by George R.R. Martin are full of them. A comma splice would look like this: "I went to the store, I bought milk."
Correcting a run on sentence is easy, but there is no one right way to do it. You have a few options. The simplest is to simply add a period: "I went to the store. I bought milk." But if you don't want the reader to make the full stop a period creates, you would use a semicolon: "I went to the store; I bought milk" or a long dash: "I went to the store—I bought milk." If you like, you could also use a conjunction: "I went to the store and I bought milk" or another joining word: "I went to the store where I bought milk."
A really long sentence is not necessarily a run on sentence. As long as you've punctuated correctly, and you have all the joining words you need, there is no limit to the length of a sentence. The only problem you run into then is confusing your reader: by the time a reader gets to the end of a really long sentence, they may forget how it began.
Remember: a run on sentence results when you try to join two sentences together without using the correct mortar. A comma is not strong enough on its own, so use a semicolon or a joining word, or just use a period and leave them separate. If you follow this, you can go on and on without running on. ;)
