Hello, Brogan: Actually, the past tenses are a very complicated issue to learn in Spanish. It would take some lessons to get to learn all of them. Here you are some help: Pretérito indefinido: Use it when the action is over and it remains in the past. With no direct relationship with the present. - Trabajé dos años de enfermero en Nigeria. -->> it means I worked some time ago for two years but so long ago (for me) that it has no influence on the present. Pretérito perfecto: Use it when the action is over but it is close to the present. - He trabajado de enfermero en Nigeria este verano. --->> The summer is over but for me I feel this experience close to me now, so I can use pretérito perfecto. **IMPORTANT: in most of Central and South American countries the "pretérito perfecto" is not used so it makes things easier. Pretérito perfecto is used mainly in Spain (although in some parts of Spain it is not used: Canarian Islands or Galicia). Did it help you? I hope so.