This research is a corpus-based study of secondary and college ESL Malaysian learner's written work by identifying and classifying the types of errors in the Past Tense Auxiliary "Be". This research studied the past tense auxiliary "be", types of past tense auxiliary "be" errors and frequency of past tense auxiliary "be" errors found in the Malaysian Corpus of Students' Argumentative Writing (MCSAW) corpus using the WordSmith Tools Version 4.0 and using the Error Analysis (EA) approach. The findings revealed that there are seven types of errors. They are Tense Shift, Agreement, Missing Auxiliary "Be", Wrong Verb Form, Addition and Misformation and Misordering. This study can be used as a guide for English Language teachers to identify the most common errors in using the Past Tense Auxiliary "Be" made by the ESL learners and decide what remedial action can be taken to prevent them from making these errors. It can also help teachers improvise and develop materials which are not only more suitable but also cater to the needs of the students. In addition to the materials, teachers can also revise their teaching approaches and strategies to ensure effective teaching and learning of these grammar components.