This fills in the grey area you may already have been wondering about: how tener gives tiene, and how querer gives quiero.
When an e sits inside the root of the verb, not in the -er ending, stressing it usually splits it into ie.
querer → quiero · tener → tiene · venir → viene
By the plain rule you would get "quero" and "tene". The accent lands on that e in the present tense, and under that pressure it splits.
It does not happen in tengo and vengo, where the n and g seem to hold it up.
The rule looks abstract until you spot it sitting between the two languages. English has a word for time that it only uses in music: tempo. Spanish for time is tiempo.
Most of the time it happens, but not always. You have already met two verbs where it does not.
For he thinks the student offers "piense", because after ie an e sounds like it matches. That is Turkish grammar, not Spanish.
pensar is an -ar verb → piensa, never "piense"
Pienso · Piensa · Piensas · ¿Qué piensas?
Asked what pensar means, the student answers "thoughtful". But there is no verb to thoughtful. The answer is to think.
Entender is the more common word for to understand and the better one to use, but comprender is easier to remember and does not split, so it is a fair fallback.
¿Qué pasa? (what's happening) · Quiero saber qué pasa · Entiendo · Entiende · Entienden · Pierdo · Lo pierdo · Los pierdo
¿Qué pasó?, with the accent on the end, is the past: what happened? That comes later.