haber

The auxiliary verb for all compound tenses (he comido, "I have eaten") — unlike ser/estar, haber is essentially never used as a standalone "to have" for possession (that's tener). Its impersonal form hay ("there is / there are") is one of the first things every learner memorizes, and it never changes for plural.

• He terminado el trabajo. — I have finished the work.
• Hay dos gatos en el jardín. — There are two cats in the garden.
• Había mucha gente en la fiesta. — There were a lot of people at the party.

Pluralizing hay to match what follows it (*han dos gatos*) — hay (and habia, hubo, etc.) always stays in its impersonal singular form no matter how many things there are; only the noun after it pluralizes.

• expression "No hay mal que por bien no venga" — A centuries-old Spanish proverb — "there's no bad from which good doesn't come" (every cloud has a silver lining) — listed in the Instituto Cervantes's Refranero Multilingüe, used repeatedly by Cervantes in Don Quijote and the title of a circa-1630 play by Juan Ruiz de Alarcón.
• expression "No hay dos sin tres" — A widely used Spanish proverb meaning that when something happens twice, a third time tends to follow — the Spanish counterpart to English's "third time's the charm," often invoked half-jokingly about winning or losing streaks.