There is no *tidak harus” in Indonesian (or rather, *tidak harus occurs rarely and only in special contexts). So how do you negate harus? A quick glance at English will help here. In English we have the terms “must” and “have to”, and we often think they have more or less the same meaning. “I have to go now” and “I must go now” are not exactly the same, but they are pretty close. It is when we transform them into negatives that we see a clear difference. “You mustn’t go” expresses prohibition, but “You don’t have to go” suggests that you have a choice.
In Indonesian, when you want to say “must not” expressing prohibition, you should use tidak boleh. But when you want to say “don’t have to” implying that you have a choice, you should use tidak usah or tidak perlu.
Anda tidak boleh masuk ke sana.
You mustn’t go in there
Anda tidak perlu/usah masuk ke sana.
You don’t have to go in there (if you don’t want to).
By the way, usah never occurs on its own. It is always a negative expression, preceded by tidak.
