Patterns in Verb Stem Changes
Pattern 1: e→ie
(-ar and -er verbs)
Model Verb: pensar

  
  Present Indicative Present Subjunctive Imperatives  
  pienso piense
---
  
  piensas pienses piensa / no pienses  
  piensa piense (no) piense Ud.  
  pensamos pensemos (no) pensemos  
  pensáis penséis pensad / no penséis  
  piensan piensen (no) piensen Uds.  
  Other tenses and forms are unaffected.  
 

Other Common Verbs With This Pattern (*most common)

acertar, apretar, ascender, atender*, atravesar, calentar, cegar, cerrar*, comenzar*, confesar, defender*, despertar*, empezar*, encender, entender*, enterrar, fregar, gobernar, helar, manifestar*, mentar, merendar, negar*, nevar*, pensar*, perder*, quebrar, querer*, recomendar*, sentar*, temblar, tender, trascender, tropezar, verter