Before the boat had been underway for ten minutes, I realized that Miss Bradley was a remarkable bore.
(1) The sentence contains a non-restrictive clause at the beginning, meaning the author does not consider the words before the comma as absolutely necessary to the sentence's full meaning. The author indicates this by using a comma after the word "minutes." (2) The sentence refers to two different times. The first part uses a past perfect tense, "had been," indicating that the boat had left before the "I" realized Miss Bradley was boring. (3)
Before (adverb) the (definite article) boat (noun) had been underway (verb) for ten minutes (adverbial phrase), I (pronoun) realized (verb) that (conjunction) Miss Bradley (noun) was (verb) a (indefinite article) remarkable (adjective) bore (noun). (4) complex sentence (one independent and one dependent clause)
The best way to learn the sentence's structure is for you to write one with the same pattern.