How do you conjugate poder and usar in the past tense in spanish?

There's two kinds of past tense in Spanish (to make it more confusing of course), there's imperfect, and preterite. Knowing when to use both can be confusing...if your saying "I played soccer," you would use the imperfect tense because it has no definite time. If you are saying "I played soccer yesterday," or "I ate lunch one hour ago," you would use the preterite because it happened at a specific time.

Imperfect endings: (ER/IR VERBS)

yo - ía     tú - ías    él/ella/usted - ía  

nosotros - íamos    vosotros - ías    ellos/ellas/ustedes ían

(AR VERBS)

yo - aba     tú - abas   él/ella/usted - aba

nosotros - ábamos     vosotros - abais    ellos/ellas/ustedes - aban

Preterite endings (ER/IR VERBS)

yo - í       tú - iste      él/ella/usted - ió

nosotros - imos     vosotros - isteis    ellos/ellas/ustedes - ieron

(AR VERBS):

yo - é      tú - aste     él/ella/usted - ó

nosotros - amos    vosotros - asteis    ellos/ellas/ustedes - aron

And finally, as if this isn't hard enough, poder is a stem changer (o-u) and a super irregular so you conjugate it with a whole different set of endings, the super irregular endings, which are

yo - e    tú - iste    él/ella/usted - o

nosotros - imos   vosotros - isteis    ellos/ellas/ustedes - ieron

So instead of "yo podí," the correct answer is "yo pude," because you have to take the stem change and the special endings into account...but only in preterite!

Conjugating usar is very basic, "yo usé," (preterite) or "yo usaba," (imperfect).

I hope this was helpful...and not confusing!

in english pick the right answer. charlie explains that you know, in south america.