"Betty failed the final test because she had not been attending class." This sentence expresses that Betty had a habit of not attending class. "had (not) been + verb-ing" shows an action that continues over a period of time before a given event: in this case the past event was "the final test." This grammar point is known as the past-perfect continuous tense. Also, the sentence implies that Betty might have taken the test, but she failed it, since she did not come to class before and it would be assumed that she was not prepared. "Betty failed the final test because she did not attend class." This sentence shows a single event in the past tense. In contrast to the sentence before, this sentence shows that Betty did not attend the class during which the test was taken and therefore failed it. Both of these sentences can be seen as very similar, but due to the tenses used, the implications of what is actually happening in each example is quite different. Also, the title for your question is a bit incorrect: Instead of writing: "how dost it use? (this sentence)" ; you should write: "How do you use these sentences?" "dost" is a very old conjugation of the verb "to do" and is no longer used in modern English. I hope this helped. Please let me know if you have any more questions.