What you already know about a sentence is that it is a sequence of words, starting with a capital letter, and ending with a full stop. When you have come to the end of this book, you will have a very different understanding of what a sentence is, and what it is made of. This understanding was traditionally called an understanding of grammar. In linguistic theory it is the understanding of SYNTAX, which is the structure of the sentence. It will require some work to achieve this understanding; here are the reasons which I think make it worth the trouble.