Question

i need help on this stem changer
tu (pensar)______ que tener un perro es una cosa buena

Answer 1

Look at your conjugations
yo- pienso
tu- piensas
el/ella/ud.- piensa
nosotros- pensamos
ellos(as)/uds.- piensan

Tu would be piensas

Answer 2

Let's also look at WHY there is a spelling or stem-change! When it makes more sense to you, and you can "trust your ear" it will get easier.

pensar (e---ie) When you pronounce the infinitive, the stress is on the end. do you know the rule about pronunciation when there is no written accent mark? If the word ends in ANY consonant, except "n " or "s" the stress falls n the last syllable. pen-SAR.

Now look at the "yo" form. How would you like the sound of PEN-so? ıecause I'm shifint the stress onto the first syllable, I'll make the "e" a bit stronger by adding "i" = It "sings" to you! PIEN-so. That is the same for all forms EXCEPT nosotros (and vosotros, if your text teaches it._There the strees is just like the infinitive: pen-SA-mos & pen-S´AIS. That is the only form with an accent mark

Sra