I have realized/realized/know that it is not right.

[Hansman]

I have realized that it is not right.

Is the sentence wrong or unnatural to you?

Should it be 'I realized that it is not right' or 'I know that it is not right'?

 

[Tarheel]

Well, you haven't provided any context. Keeping that in mind, "I know it is not right" seems to be most natural. Now, if I had some context to go by it's possible that I would provide you with a different answer.

 

[emsr2d2]

You've presented three context-free sentences:

1. I have realised that it is not right.
2. I realised that it is not right.
3. I know that it is not right.

None of them is more correct or natural than the others. They're three grammatically correct sentences. Now it's up to you to provide context for all three, explaining why you would use the present perfect in #1, why you used the simple past and stuck with the verb "realise" in #2, and why you used the present tense and changed the verb to "know" in #3. Over to you!