The Spanish verb pensar (to think) is semi-regular since its stem changes in some forms in El Presente but it has the same regular endings as other regular ‑ar verbs:
|él / ella / Ud.||piensa|
|nosotros / nosotras
|vosotros / vosotras
|ellos / ellas / Uds.||piensan|
The first vowel -e- in the stem becomes -ie- except for the nosotros and vosotros forms.
Look at and listen to these examples: