Which conditional verb forms proved most difficult for Thai secondary school students to produce, and what errors resulted in written and spoken English, were studied. Data was collected from two tasks: 1) a gap-fill task by 68 twelfth-grade students in an integrated English program at a public high school in Bangkok, Thailand and 2) a spoken task to explore the errors made by 20 students from the same group. Results of the gap-fill task were that the majority of participants misused the subject and verb agreement in Factual Conditional; present simple tense in Future Predictive, together with Present Counterfactual; and future simple tense in Past Counterfactual. In the spoken task, the future simple tense was most commonly misused in the target conditional types altogether. Pedagogical implications were provided in the study. For example, teachers of English should have their students effectively master grammatical if-conditional constructions, especially the present simple tense, future simple tense, and even the subject and verb agreement in the production of conditional sentences.