This is grammatical:
- I'm too tired to drive.
but this isn't:
- I'm tired to drive.
Why? How can removing an adverb make a sentence ungrammatical?
The primary sense of too is to indicate a higher degree than the maximum that is consistent with meeting some condition, achieving some purpose, actualising some situation:
[35]
- i. She was too tired to continue.
i. She was too tired to continue.
- ii. We didn't go out: it was too wet.
ii. We didn't go out: it was too wet.
In [i ] the degree of tiredness was greater than the maximum consistent with her continuing: the sentence thus entails that she didn't continue. In this sense, too licenses an indirect complement with the form of an infinitival clause or a for phrase (too valuable for this kind of use).
This indirect complement indicates the condition, purpose, or potential situation, but does not have to be overtly expressed. In [ii ], for example, there is no complement in the wet phrase, but we understand "too wet to go out".
1.ii. She's [too young to go to school]. -- (indirect complement)
1.iii. She's [young] to be going to school. -- (adjunct in clause structure)
. . .
In [ii ] the infinitival is a constituent of the AdjP, but is licensed by too rather than by the adjective young. It is therefore an indirect complement; . . .
While [ii ] says that she is young to a degree higher than that at which she can or should go to school, [iii ] says that she is young relative to those who go to school: it is unexpected or noteworthy that someone as young as she is should be going to school.
