i need help on this stem changer

tu (pensar)______ que tener un perro es una cosa buena

Look at your conjugations

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

Tu would be piensas

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

To conjugate the verb "pensar" (to think) in the second person singular (tu), you need to replace the -ar ending with the appropriate stem change. In this case, the stem change in pensar is an e to ie change. Here's how you can conjugate it:

1. Start with the stem of the verb: pens-
2. Replace the e with ie in the stem: piens-
3. Add the appropriate ending for the second person singular (tu): piensas

So the correct conjugation of "pensar" in the second person singular (tu) is "tú piensas." Putting it into the sentence you provided, it would be:

"Tu piensas que tener un perro es una cosa buena." (You think that having a dog is a good thing.)