In the paragraph that begins on the bottom of
page 2 and ends on the top of page 3, why does
the author say that brain injuries are "almost
impossible to detect"?

it is because it is the hardest to operate

Explanation:

the sentence does not violate the phrase structure second rule (introduced by noam chomsky) because noun phrases consist of a determiner such as a, the or every, and a noun. in the sentence, the noun phrase the cake includes a determiner (the) and a noun (cake).

on the other hand, the first rule indicates that a sentence encloses a noun phrase, followed by a verb phrase. the example sentence can be analysed as follows:

john (n) + ate (v) + the (det) + cake (n).