"I saw them when they were here."
Translation:Los vi cuando estuvieron aquí.
I used fueron instead of estuvieron - could someone help me understand the difference?
When it comes to location, we use "estar":
Mary is here / Mary is in Barcelona / Mary was at Tom's: Mary está aquí, Mary está en Barcelona, Mary estaba/estuvo en casa de Tom.
Dan, Another thing to be aware of is that you used the preterite of ser. You needed the imperfect--of estar--because they were here implies an action whose beginning and end are not taken into account, according to Larousse All Spanish Verbs from A to Z. Sometimes it is tricky to figure out if the preterite or imperfect tense should be used.
Duo suggests the preterite here. The imperfect would be for a sentence like "They used to be here". Although the beginning and end of their visit is not specified in this sentence, it is still a definite, isolated event, and not a vast vaguely defined time period.
Estuvieron or estaban are both acceptable here precisely BECAUSE you said there is no context offered here. Maybe the used to live there for a solid 6 years and "I saw them while they were here".
I answered Yo les vi cuando ellos estuvieron aquí. and it was accepted, but the suggested solution uses ¨los¨. Is ¨les¨ really also correct?