The increasing role of post-editing as a way of improving machine translation output and a faster alternative to translating from scratch has lately attracted researchers’ attention and various attempts have been proposed to facilitate the task. We experiment with a method to provide support for the post-editing task through error detection. A deep linguistic error analysis was done of a sample of English sentences translated from Portuguese by two Rule-based Machine Translation systems. We designed a set of rules to deal with various systematic translation errors and implemented a subset of these rules covering the errors of tense and number. The evaluation of these rules showed a satisfactory performance. In addition, we performed an experiment with human translators which confirmed that highlighting translation errors during the post-editing can help the translators perform the post-editing task up to 12 seconds per error faster and improve their efficiency by minimizing the number of missed errors.