I need an example of a sentence using the word oath.
An oath is something like a promise. The major difference between the two is that an oath is much more solemn. You would not make an oath that you and your friend will have lunch together tomorrow. That would be a promise. However, when something much more important is being promised, that is an oath. That is, for example, why a new president takes an "oath of office" before he (or someday she) takes power.
So, you can make a sentence about any sort of solemn promise like that. For example, "At many schools, students take an oath to obey certain rules like rules that forbid them from copying their homework."