I've seen many beginners struggle with using the word 너. This word 너 means "you," but it can actually sound (very) rude.
When used appropriately, 너 is a normal word. However, it can't always be used alongside regular casual speech - there are some specific conditions where 너 can be used.
Here's a full explanation of how to use 너, what 너 means, and when to use 너.