Simple tenses (meaning the ones that are not compound
tenses) are used the same way as in English. The modal
verb is conjugated (put into the right tense) and the
full verb is appended in infinitive (unchanged) form.
hacerlo. = He wanted to do it. Nosotros podíamos
comerlo. = We were able to eat it. Vosotros podíais
nadar. = You were able to swim.