Ninety percent of communication takes place in the present tense. For that reason book I was dedicated to the present tense. You learned subject/verb sentences, verb plus present participle sentences and conjugated verb plus infinitive sentences.
The past tense is covered in book two. It is difficult because there are two past tenses, the preterit and the imperfect. For that reason book II was dedicated to the past tense.
Book three presents the future, conditional and the four perfect tenses. When you complete book three you will have learned all the tenses that the average Spanish person uses. You will know nine tenses.
As mentioned above, ninety percent of communication takes place in the present tense. Why is it necessary to learn all nine tenses? Because you have no control over of what the other person is going to say. Knowing all the tenses will enable you to under-stand what you hear.