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!