Hello The words thereabouts, therein, thereof, thereby, thereafter, therefore, thereon, etc. are adverbs and, I must add, rather formal adverbs. Some of these are used in legal matters.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
As a linguist, I would say that it depends on the context in which you are trying to speak. In writing, especially formal writing, it is best to use what is called "prescriptive grammar (prescribing a way in which someone should speak)," which means "correct grammar." However, the prescriptive rules are more or less arbitrarily chosen. The rule that prepositions cannot end a sentence comes fro