Hola Keila

The correct form is "vivía" because it is in the imperfect tense, which is used to describe ongoing or repeated actions in the past. The sentence "Su tío vivía en Uruguay cuando era joven" means "His/Her uncle used to live in Uruguay when he was young". The imperfect tense "vivía" indicates that living in Uruguay was a continuous or repeated action during his youth.

Using "vivió", on the other hand, would imply a single, completed action and would not convey the same sense of an ongoing or habitual action in the past. "Vivió" is in the preterite tense, which is used for actions that were completed at a specific point in time. Therefore, saying "Su tío vivió en Uruguay cuando era joven" would mean "His/Her uncle lived in Uruguay when he was young", but would suggest that it was a specific, perhaps shorter, period rather than an ongoing state during his youth. This does not fit the context of describing a continuous period in the past, which is why "vivía" is the correct choice.

I hope this clarifies your question. Feel free to ask if you have any more doubts!

Hasta pronto

Silvia