Breakdown of Prefiero que mejores tu postura.
tu
your (singular)
que
that (relative pronoun, conjunction)
preferir
to prefer
mejorar
to improve
Questions & Answers about Prefiero que mejores tu postura.
Why is it mejores and not mejoras here?
Because the sentence uses prefiero que (I prefer that). When you express a preference or wish and the subject changes from one person to another, Spanish requires the subjunctive mood. Mejores is the present subjunctive form of mejorar (to improve) for the tú form.
Could I just use the infinitive and say prefiero mejorar tu postura?
No, because there are two different people involved: the person preferring is not the same as the person improving. You only use the infinitive after preferir if the same person is doing both actions. For example, prefiero mejorar would mean that I prefer to improve my own posture.
Why does preferir become prefiero?
Preferir is a stem-changing verb. In the present tense, the second e changes to ie for most forms, including the yo form (prefiero). Remember that the nosotros (preferimos) and vosotros (preferís) forms do not use this stem change.
Why doesn't the word tu have an accent mark in this sentence?
In Spanish, tu without an accent means your, which is a possessive adjective used here for your posture. If it had an accent mark (tú), it would mean the subject pronoun you.