Probably the easiest way to fix a run-on sentence is to separate its ideas into two distinct sentences. For example, "I made a sandwich it tasted great" is a run-on sentence because it contains two subjects ("I" and "it") and two predicates ("made a sandwich" and "tasted great") without proper separation. Fix it by writing it as follows: "I made a sandwich. It tasted great." Make sure to put a period at the end of both sentences.
You can use a semi-colon to join two distinct thoughts. The key here, however, is that the sentences must be related. It is okay to fix the example above as "I made a sandwich; it tasted great," because the two thoughts are closely related. However, it would be incorrect to fix the following run-on sentence with a semi-colon, because the thoughts do not flow together: "I went to school it was a rainy day."
Use a comma and a connecting word to link two thoughts without making them into separate sentences. Connecting words are small words such as, "and," "but," "for" and "even though," among others. For example, fix "I went to school it was a rainy day" by rewriting it as, "I went to school, even though it was a rainy day." Fix "I ordered the soup it was delicious" as, "I ordered the soup, and it was delicious."
Semi-colons must be used with different connecting words, such as "therefore," "however" and "furthermore." They are always followed with a comma. Fix "I wanted to sleep it I had to go to school" by rewriting it as, "I wanted to sleep in; however, I had to go to school." Fix "I wanted a second helping I took one" as, "I wanted a second helping; therefore, I took one." Note that these examples could also be rewritten with commas and the connecting words "but" and "so," respectively.