Try and think about the use of tenses in the three following cartoons.

Source: http://bizarro.com



police_dogsUse the preterit when the action is over. "I thought", but won't think so anymore.

police_rights_readingNote the construction of the passive form:   subject + auxiliary be + past participle

"be" will be conjugated and the main verb will be invariable. In this example, if the last part of the sentence was in the preterit, the police officer would say "... anything you say was (preterit form of be) used against you".

police_tensesNote the use of the semi-modal "have to" at the end of the sentence. It could easily be replaced by the modal MUST. However, if HAVE TO can express an obligation resulting from outside like laws or rules, then MUST will rather express a personal necessity, or the feelings of the speaker. For example: I must stop smoking.