question

have to / has to - 一定要

I shall leave tomorrow. 我明天會離開
I will leave tomorrow. 我明天會離開 (when you use 'I will' instead of 'I shall', 有較肯定的意思)
I will have to leave tomorrow. 我明天一定要離開 (更加肯定)

In general, I and We should use 'shall' in future tense. As above mentioned, I will and We will 比 I shall and we shall 較肯定.

I will leave 我將會離開
I will have to leave 我將需要離開
相信上開是談話式:
1/ 我將會離開 - 沒人會說 I will leave. 通常會說 I have to leave (before 11 o'clock)
2/ 我將需要離開亦同樣可說 I have to leave (very soon)
Why? 因 I have to leave 肯定是 future tense, 故不需用will. 例如:
Tomorrow is Saturday 而不會說 tomorrow will be Saturday. 因今天是 Friday, 明天肯定是 Saturday.

I will leave.---->我會走

例句:I will leave tomorrow.我明天會走。

I will have to leave.---->我將要走

例句:I will have to leave at ten tomorrow.我將要在明天十時走。

I will leave 解 我將會離開
而 I will have to leave解 我將需要離開

Hope it's helpful to u.