Rewrite each original sentence in order to make it easier to understand by non-native speakers of English. You can do so by replacing complex words with simpler synonyms (i.e. paraphrasing), deleting unimportant information (i.e. compression), and/or splitting a long complex sentence into several simpler ones. The final simplified sentences need to be grammatical, fluent, and retain the main ideas of their original counterparts without altering their meanings.
Q: Probably in the ninth century, the village became part of the territory of the monastery of Reichenau.
A: 
it's thought would become part of the monestery of Reichenau in the 9th century.