This is the spice to season the rice with. 

【翻译】这是用来调制米饭的调味品。

 the spice 是“调味品”， to season the rice with 不定式作定语。具体说，the spice 是with 的逻辑宾语，意思是“用其调制米饭”。

This is what to season the rice with. 

跟上句相比，用what 表示 the spice （调味品），那意味着what 是“复合型关系代词”（也叫融合型关系代词），此时没有疑问概念。

如果把句子改为名词性从句，更好：

This is what we season the rice with. 

这就是用来给米饭调味的香料。不定式作定语。

(1)This is what to season the rice with. 

(2)This is the spice to season the rice with.(正确）（带有不定式作后置定语的名词短语作表语）

(3)This is what we seaons the rice with（正确）（what引导的表语从句）

(4)What to season the rice with is unclear.(正确）（疑问词+不定式作主语）

(1)疑问代词what, which, who,whom

(2)疑问副词：when,where,why,how

解答如下：

能与不定式连用的通常为疑问词，而不是复合型关系词（包括复合型关系代词what=the thing that/which和复合型关系副词where=the place where;when=the time when）

疑问词+不定式=相当于一个不定式短语可以在句中充当主语，宾语，表语等

【1】作主语

What to do is not decided yet.

该做什么还没有确定。

When to start has not been decided.

何时动身尚未决定。　

How to begin is more difficult than where to stop.

如何开始比到哪里停止还困难。

【2】作宾语

Do you know when to start?

你知道什么时候开始吗？

He didn’t know how to answer her.

他不知道该怎样回答她。　　

I showed her which button to press.

我告诉她应该按哪一个按钮。

I don’t know who to ask advice from.

我不知道应向谁征求意见。　　

Would you please advise me which to buy?

你能否告诉我该买哪一个才好?　　

Please explain (to me) where to begin and how to do it.

请 (给我)解释该从哪里开始及该怎么做。　　

【备注】有时还可用于介词后作宾语。

I found a book on how to avoid having a heart attack.

我找到一本书谈如何避免心脏病复发。

She was worried about how to fill her leisure time.

她为如何打发她的闲暇时间而发愁。

They exchanged views on the question of whom to elect.

他们就该选谁的问题交换了

【3】作表语

The question is how to carry out the plan.

问题是怎样执行这个计划。

The difficulty was how to cross the river.

困难在于如何过河。