This tool will allow you to conjugate spanish verb infinitives. It is ideal for studying as well as completing high school and college spanish class assignments. To conjugate a regular verb in the imperfect tense in Spanish, simply remove the infinitive ending (-ar, -er, or -ir) and add the imperfect ending that matches the subject. Spanish Imperfect Past tense verb endings. Now that you have some idea of when to use the imperfect past tense, I’ll let you in on some good news.