This sentence stands for the preterite perfect tense. This tense is usually used in formal written texts. We use it to describe an action in the past that happened immediately before another action in the past. So he bebido matches the first person singular of the verb beber in this tense. Finally:
<em>Yo </em><em>he bebido</em><em> toda la leche en el refrigerador (I have </em><em>drunk</em><em> all the milk in the refrigerator)</em>
