Vím is the first person singular form, while nevědí is the irregular third person plural form for the verb vědět. See, for example: https://en.wiktionary.org/wiki/v%C4%9Bd%C4%9Bt#Conjugation, for the full conjugation. (Click on Show if the table doesn't immediately appear.)
This has been asked quite a few times before, like https://forum.duolingo.com/comment/25071408/I-do-not-know-whether-to-look
If you search you will find more.