What does the authour mean by language as a structure?``grammar taught me language as a structure. I even learned the elementary categories of...

The author is talking about the ways in which language
organizes or structures how words are put together to convey messages. When you want to
ask someone to pass you the textbook, you don't say the words in any order that comes to
mind; "Textbook me please to the pass" makes no sense because there is a specific order
in which words should be presented to be understood. That order is part of the structure
of language.

Another aspect of the structure of language is
what your quote refers to as the "categories of philosophy." Some parts of language
change in response to different types of use; I see, they
saw, he has seen, she
will be seeing. These differing forms of the same basic
verb (to see) are examples of using the structure of
language to correctly communicate the message being presented.