The two most common Spanish conjunctions are y (translated as "and") and o (translated as "or"). The spelling and pronunciation of each may can change based on the word which follows the conjunction. The purpose of the change is to clearly differentiate the conjunction from the word which follows.

The conjunction o changes to u before words beginning with o or ho. The conjunction y changes to e before words beginning with i or hi. The conjunction y does not change before words, such as hierba, that begin with the ia, ie, io or iu sound, regardless of spelling.

o changes to u

Examples:
No recuerdo si habían siete u ocho manzanas.
I don't remember if there were seven or eight apples.

No recuerdo si era belga u holandés.
I don't remember if he was from Belgium or Holland.

Crees que Andrés tiene diez u once años.
You believe Andres is 10 or 11 years old.

Ella tiene amor u odio.
She has love or hate.

Nosotros somos soldados u oficiales.
We are soldier or officials.

Ellos cantan u oran.
They sing or pray.

Hay nueve u ocho tomates.
There are nine or eight tomatoes.

Las novelas están llenas de egoísmo u odio.
The soap operas are full of selfishness and hatred.

Creen que la niña comió ocho u once bananas.
You think the girl ate eight or eleven bananas. 

y changes to e

Examples:
Juan es muy guapo e inteligente.
Juan is very handsome and intelligent.

Yo era bueno e invencible.
I was good and invincible.

Las novelas son largas e intrigantes.
The soap operas are long and intriguing.

Tú eres bueno e incapaz de una maldad.
You are good and incapable of doing evil.

Nosotros estábamos cansados e irritables.
We were tired and irritable.

Ella tomaba café con María e Isabel.
She drank coffee with Maria and Isabel.

Ustedes son testarudos e imposibles.
You are stubborn and impossible.

Ellos recogen e incineran la basura.
They pick up and incinerate the trash. 

Rosa sonríe e imita al payaso.
Rosa smiles and imitates the clown.