Fortunately for us gringos who are learning the Spanish language, there are only a few "weiner verbs" that we might have to use in our daily conversations as we're starting out.  The ones that come to mind are DORMIR, PEDIR, SERVIR, and VESTIRSE.   There are some others you might need to know, depending on the situations you put yourself in. . . .  

Let's look at a couple of three of these verbs and see how their conjugated in the PRETERITE TENSE.  You should see why we call them "weiner verbs" -- in spite of my lousy drawing skills!! -- when you look at the stem-change that takes place in the third person singular and third person plural ( that's the él, ella, usted and the ellos, ellas, ustedes for the grammatically-challenged among us! ):