This study was conducted to identify Iranian EFL teachers' perceptions regarding the most serious written errors made by young adult EFL learners, along with the most effective types of written corrective feedback to deal with them. Data were collected from 253 Iranian EFL teachers through a questionnaire made by the researchers and were analyzed using descriptive statistics. The results showed that the most serious errors perceived by the teachers were verb form and verb tense, and word order driven by psychological verbs. The most effective feedback types to treat these errors were (a) direct, non-negotiated feedback and (b) indirect, negotiated. The findings contribute to the research concerning the most frequently noticed errors by EFL teachers along with the most frequently employed corrective feedback strategies to address them.