Final answer:
In the present tense, some -er and -ar stem-changing verbs undergo stem changes, but this is not the case in the preterite tense. One example is the verb 'pensar' (to think).
Step-by-step explanation:
In the present tense, some -er and -ar stem-changing verbs undergo stem changes, but in the preterite tense, these stem-changing verbs do not change.
This statement is true. One example of an -ar verb that undergoes stem changes in the present tense but not in the preterite tense is 'pensar' (to think). In the present tense, it changes from 'pienso' (I think) to 'piensas' (you think) for the second person singular form. However, in the preterite tense, it remains 'pensé' (I thought).
In the present tense, stem-changing verbs such as 'pensar' (to think) change from 'e' to 'ie' (él piensa), but in the preterite tense, these verbs do not undergo a stem change (él pensó).
It's important to note that this general rule applies specifically to -er and -ar stem-changing verbs; some -ir stem-changing verbs do have a stem change in the preterite, but in a different way (e.g., 'sentir' becomes sintió in the preterite).