> This is grammatical:

> > 1. *I'm **too** tired to drive.*

> but this isn't:

> > 2. *I'm tired to drive.*

> Why? How can removing an adverb make a sentence ungrammatical?

Your example #1 means that it is expected that you will **not drive**. And that the reason why it is expected that you won't drive is that you are **too tired**. Thus, your example #1:

* 1. *I'm **too** tired to drive.* -- (OP's #1)

The adverb "too" is part of the adjective phrase "too tired to drive":

* *I'm [**too** tired to drive].*

where the adjective phrase "too tired to drive" is headed by the adjective "tired".

The adverb "too" is needed because it is what indirectly licenses the infinitival clause "to drive". Without the presence of "too", there is no licensor for that infinitival clause "to drive".

Your example #2, which is missing the licensor "too", doesn't seem to make much sense in the usual types of context. (Though, perhaps an unusual context could be thought up to make #2 acceptable.) If a different adjective is used instead of "tired", then sometimes it will create an acceptable example, e.g. *"I'm **willing** to drive"*, which means that it is possible that I might end up driving, for I probably won't refuse a request for me to drive.

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Here's some info from a vetted grammar source, the 2002 *CGEL*.

Page 585:

> 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:

> [25]

> * i. *She was too tired to continue.*

> * 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".

Notice that for [35.i ] *"She was too tired to continue"* entails she didn't continue. That means that she did **not** continue.

Page 1256-7:

> 1.ii. *She's [too young **to go to school**].* -- (indirect complement)

> 1.iii. *She's [young] **to go 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. 

Notice that for [1.ii ] *"She's too young to go to school"* means that it is expected that she did **not** go to school.