一定【yídìng】

一定 goes after the subject and before the verb or adjective. With what you can't see, it is a confident conclusion: "must be".

Before an adjective there is no 是.

• × 他一定是很忙 → 他一定很忙

一定要 for "be sure to"

With 要, or with 会 for a promise, 一定 is about will: an insistence, a request or a commitment.

不一定 and 一定不

Where 不 goes changes the meaning. 不一定 is "not necessarily"; 一定不 is "definitely not".

不一定 can stand alone as a reply: "not necessarily".

As an adjective before a noun, 一定的 means "a certain amount of": 一定的影响 is "some effect".