We went in search of the world’s hardest language
The Economist

I have to point out that “verb-endings on statements to show how the speaker knows something” isn’t unusual. It’s not available in English, but Japanese language can do it. I can say that Japanese language does it even better because you can add “verb-endings on statements to show the assumed relationship between the speaker and the listener”. In some Japanese detective stories, this can be the clue to show that the speaker and listener have a closer relationship than they claimed to be.

For example, “the boy played soccer (I know because I saw him)” can be “otoko-no-ko (the boy) ha soccer de asondeitayo (played-knowledge)” . This is a “neutral speaker speaking to an equal social status and familar but not too familiar listener”. Changing the last to “asondeimasitayo” (played-polite-knowledge) is for a “neutral speaker speaking to a higher social status and/or non-familar listener” (maybe a police officer asked “have you seen this boy?”). Changing the last to “asondetakedo” (played-casual-knowledge) is for a “speaker speaking to a lower/equal social status and familar listener” (maybe another boy asked a classmate “have you seen this boy?”).

Using these 3 examples, you would expect that a butler will use 2nd form to speak to the lady that he is serving. But he may use 3rd form instead because the butler and lady had been childhood friends and didn’t notice that someone couldn’t be listening.