The research focuses on the understanding of 20 students in SMP N 9 Surakarta class one towards negative form in grammar based on tenses. There are three tenses applied in this research; present continuous tense, simple present tense and past tense which is divided into two (regular verbs and irregular verbs). Besides that, there are also two factors that could influence their successful mastering English; private course and interest.
The purposes of this study are to find out the successful of students mastering English trough their understanding of negative form in every tense and the factor which affects and motivates them.
The research employs a descriptive-qualitative method. The data were collected by employing a purposive sampling technique. The research was conducted by using questionnaire. The researcher collected the data from the answer of 20 students as the result of questionnaire.
The result of the data analysis shows that: 1) There are 2 types of negatives in this research; auxiliary and to be. The most difficult type of negatives for students is auxiliary. 2) Most students made mistakes in simple present tense and past tense (irregular verbs). The second mistakes were made by students in past tense (irregular verbs) only. 3) Private course helps students increase their knowledge about English but it does not really affect the student to get better score than those who do not take private course. The most influent factor is interest. All students who got perfect score are interested in English and those who are interested in English got better score than those who are not interested.
It can be concluded that the most difficult type of negatives is auxiliary. It is proved by the most mistakes made by students in simple past tense (irregular verbs) and simple present tense. They still cannot apply the right auxiliary in English and they have limited vocabulary, especially V2. The most influent factor towards the successful of students understanding English is interest.