Is “He is too weak that he cannot walk” a correct sentence?

Question

Consider the following:

1. He is too weak that he cannot walk.

2. He is so weak that he cannot walk.

3. He is too weak to walk.

I feel all the above sentences are correct. But my grammar book suggests, that the first one is wrong and the rest is correct. Why is it so? Any explanation?

Answer 1

Valid constructions:

• too <adjective> to <verb>
• so <adjective> that <condition/state expressed as a standalone sentence>

There is no such construction "too <adjective> that <condition>".

Answer 2

2 and 3 are perfectly right, 1 is wrong. The correct formation would be "He is too weak and cannot walk". The formation of such a sentence requires an implied comparison - "He is so weak that he cannot walk" implies a comparison to someone/something that is extremely weak and cannot walk, while "He is too weak that he cannot walk" just says he is too weak - doesn't show the he is as weak as someone/something else that is extremely weak and cannot walk.