SER, TENER, or HABER? 1

Ser — to be (identity and permanent traits) — Ser expresses identity, origin, material, profession, and time: Soy médico. Es de México. Es de oro. Son las tres. It is used for inherent characteristics that define what something or someone is, not their current state. Also for passive voice: fue escrito por Cervantes.

Tener — to have, and dozens of expressions — Tener expresses possession (Tengo dos hijos) and obligation (Tengo que estudiar). Crucially, many Spanish expressions use tener where English uses "to be": tener hambre/sed/frío/calor/sueño/miedo/razón/prisa/celos. These must be memorised as tener idioms.

Haber — auxiliary and hay — Haber forms compound tenses: he comido, había llegado, habrá salido. As hay it means "there is/are" and is invariable: Hay un problema. Hay muchos estudiantes. Never use ser/estar for existence — hay is the only correct form for "there is/are".