Learn Finnish: Etkö sinä laulele, kun olet rento? - Don't you sing when you are relaxed?

The base verb is laulaa (to sing). By adding the frequentative suffix -ella, it becomes laulella, which means to sing casually, hum, or sing without a specific goal. This perfectly fits the relaxed (rento) mood of the sentence.

Why does the verb stem change to laule- before the suffix?

When adding the -ella or -ellä suffix to verbs ending in -aa or -ää, the final vowel changes to an e. That is why the base verb laulaa drops its final a and adds the suffix to become laulella.

Why does the sentence use laulele instead of the regular "you" form laulelet?

The word etkö contains the negative verb et (you don't). Whenever you use a negative verb, the main verb must be in its connegative form. For laulella, you find this form by taking the "I" form (laulelen) and dropping the -n, leaving you with laulele.

Could I use rentoutunut instead of rento here?

Rento describes a general personality trait or a natural state of being laid-back and casual. Rentoutunut comes from the verb rentoutua (to relax) and specifically means "having relaxed" after being stressed or tense. Rento is the better choice here to describe a naturally easygoing mood.