The aim of the study was to analyze the common mistakes made by Non-English Department students in understanding Simple Present Tense and Present Continuous Tense sentences. The students who joint with the class were from Telecommunications Engineering Study Program, Institut Teknologi Telkom Purwokerto in Academic Year 2018 – 2019.  This study involved 3 classes that contained 89 students.  They were given 5 positive sentences and they had to make those sentences into negative and interrogative sentences.  They had to finish doing the test in 20 minutes.  The study showed that the students had made the mistakes in negative and interrogative sentences.  They did not use the auxiliary verbs do, does, and be appropriately.  Besides, they made mistakes when the simple present tense sentences were preceded by subject he, she, or it.  Furthermore, the use of present participial verbs was still commonly found in the simple present tense sentences so that the constructions became incorrect.  For this reason, teaching grammar in non-English Department study program had to be designed in such a way that they students could understand how to make statement sentences into negative and interrogative ones correctly.