What's the difference when we use these expressions?
I Believe You means that we trust what the person is saying is true.
We might not have tangible proof, but we believe what they're saying is true.
I Believe In You has a different meaning.
We use this expression when we have confidence in someone's ability.
For example, "I know you'll pass your English exam. I believe in you!"
Or, "I know that you'll get first place when you run the race. I believe in you. You can do it!"
You can also believe in yourself.
For example, "I've been working really hard lately, so I know that I'll be successful one day. I believe in myself."