There are three types of stem-changing verbs in Spanish that undergo changes in the present tense. Type 1 changes e to ie, such as entender to entiendo. Type 2 changes e to i, such as servir to sirvo. Type 3 changes o to ue, such as almorzar to almuerzo. The verbs querer and preferir are examples that can express desires, preferences, and be used with an infinitive like querer estudiar. Pensar is another useful stem-changing verb that means to think, think about, think of, and when used with an infinitive expresses planning to do something, such as pensar escribir.