Meaning Changing Verbs
The preterite and imperfect can affect the meaning/translation of certain verbs markedly.
Verb Preterite [Act completed/attempted] - - Imperfect [State or potential]
Conocimos a Elena ayer. - - Conocíamos a Elena ya.
'We met Elena yesterday.' - - 'We knew Elena already.'
Supimos la verdad. - - Sabíamos la verdad ya.
'We found out the truth.' - - 'We knew the truth already.'
María tuvo una idea. - - María tenía una idea.
'Mary got an idea.' - - 'Mary had (possessed) an idea.'
María tuvo una carta. - - María tenía una carta.
'Mary got/received a letter.' - - 'Mary had (possessed) a letter.'
Pudieron hacerlo. - - Podían hacerlo.
'They managed to do it.' - - 'They were capable of doing it.'
[succeeded in doing it] - - [may have done nothing]
Quisimos abrir la ventana. - - Queríamos abrir la ventana. 'We tried/attempted to open the window.' - - 'We wanted to open the window.'
[but may have done nothing]
No quise estudiar. - - No quería estudiar.
'I refused to study.' 'I didn't want to study.'
[but may have studied anyway]
The preterite form implies an act or at least an attempt to act in each of the cases above.
The imperfect implies noting more than a state of affairs or an intention.