This is a useful source of information on English grammar. However, it seems to have been designed by a committee, and none of the contributors bothered to read the whole thing once finished. As a result, the book is full of clerical errors (not typos, the spellchecker having obviously taken care of that) such as intruder words left over from sentence editing, chunks of text copied from other sections without being properly edited to fit the new context, and even grammatical puzzles like this sentence: "If you knew you had a test today, you should have studied harder". This is probably OK as an example of mixed conditional that could make perfect sense in some contexts, but it is given as an example for a rule that states: "To form the third conditional, we use the past perfect tense for the if conditional clause, and would have + the past participle of the verb for the hypothetical outcome.
(As with the second conditional, we can also use could or might instead of would. Additionally, we can use should have + the past participle to describe an outcome that ought to have happened.)" What is a learner of English supposed to make of it?