Compraste is the conjugation of the <em>ar regular verb</em> comprar for the second person singular in the preterite tense. The preterite tense is one of two simple past tenses. It is used to describe actions that took place or were completed at a certain point in the past. If you want to conjugate a regular verb in the preterite tense, you must remove the infinitive ending (in this case ar) and add the preterite ending that matches the subject. For the second person singular this ending is aste. So:
<em>Tú </em><em>compraste</em><em> la última lámpara de la tienda.</em>
