This is an irregular verb with a spelling change. It follows the conjugation pattern of the verbs abrazar and volar.
PresentPresente
Si lees sin luz, esfuerzas la vista.
If you read without light, you strain your eyes.
ImperfectPretérito imperfecto
La cantante esforzaba la voz en cada concierto.
The singer strained her voice at every concert.
Negative ImperativeImperativo negativo
Por favor, no esfuerces la vista con poca luz.
Please don't strain your eyes in low light.