More specifically, it is the Simple Past tense. This tense is used to talk about actions that took place at a specific time in the past. This action started and finished in the past, not continuing until the present. In the sentence we are analyzing here, the verb is "built", which is the past form of "build". "Build" is an irregular verb, which means its past and past participle forms do not follow the general rule. Regular verbs, on the other hand, receive -d, -ed, or -ied to form the past - for example: dance --> danced.
You can help your fellow classmates to follow the rules by reminding them of the rules and by setting an example for them by following the rules yourself.
