The aim of this research was to measure the students’ comprehension and its causes the student’s ungrammatical in using the affixation s, es and ies in the bare infinitive of the simple present tense. This research also aims to investigate the difficulties faced by the students in using affixation in the bare infinitive of the simple present tense. For collecting the data, the researcher used test as the instrument. The data collection, the researcher had given 30 questions about the simple present tense and a question about the difficulties faced by students. The students had been given 30 minutes to answer the question. The data used descriptive analisys technique. The data were conducted to 39 C Class students at the English Language Education Department of UIN Walisongo Semarang in the Academic Year of 2019/2020. The result of the research shows that the students comprehension of using affixation s, es and ies categorized as “Fair”, with a total score of 69%. The data shows that only fourteen students got “Excellent” with the percentage of 86 – 100%, four students got “Good” with
the percentage of 76 – 85%, then they were three students got “Fair” with the percentage 66 – 75%, nine students got “poor” with the percentage of 56 – 65% and the last they were nine students got “Very Poor” with percentage of 0–55%. The recomendation of this research is the students can learn more about how to use the affixation s, es and ies in the bare infinitive of simple present tense.