prevenir

to preventirregular-ir verb

Where it breaks the pattern

prevenir follows venir: irregular first-person form
prevenir follows venir: e → ie when the stem is stressed
prevenir follows venir: irregular preterite stem; the 1s and 3s endings are unstressed, so they carry no written accent
prevenir follows venir: irregular future stem; the endings themselves are regular
prevenir follows venir: conditional shares the irregular future stem
prevenir follows venir: present subjunctive is built from the irregular yo form
prevenir follows venir: imperfect subjunctive is built from the third-person preterite stem
e → i outside the present, where the stem is unstressed

Indicativo: prevenir conjugation

	Presente	Pretérito	Imperfecto	Futuro	Condicional
yo	prevengo	previne	prevenía	prevendré	prevendría
tú	previenes	previniste	prevenías	prevendrás	prevendrías
él / ella / Ud.	previene	previno	prevenía	prevendrá	prevendría
nosotros	prevenimos	previnimos	preveníamos	prevendremos	prevendríamos
vosotros	prevenís	previnisteis	preveníais	prevendréis	prevendríais
ellos / Uds.	previenen	previnieron	prevenían	prevendrán	prevendrían

Participio: prevenido · Gerundio: previniendo

irregular form

Real sentences with the conjugated form highlighted, from Tatoeba, licensed under CC BY 2.0 FR.

¿Por qué nadie me previno?
Why didn't anyone warn me?
Que no previene nuestra muerte es evidente, porque todos moriremos.
That it does not prevent our dying is evident, because we all die.
Tom previno a Mary de que John intentaría pedirle dinero prestado.
Tom warned Mary that John would try to borrow money from her.

These verbs are venir with a prefix, and they inherit its irregular forms.