Learn Spanish (Spain): No sé la respuesta.

Sé is the "I" form (first-person singular) of the verb saber (to know). It is highly irregular. While most verbs follow a predictable pattern for the "I" form by ending in -o, saber changes completely to sé. The other present tense forms of saber are regular (like sabes and sabe).

Why does sé have an accent mark?

The written accent mark is used to distinguish the verb sé (I know) from the pronoun se (himself, herself, themselves). You might have seen se in reflexive verbs like llamarse. They sound exactly the same when spoken, so the accent tells us it is the verb.

Why doesn't the sentence say Yo no sé?

In Spanish, the subject pronoun yo (I) is usually left out because the verb form sé already tells us exactly who is doing the action. Saying Yo no sé la respuesta is not wrong, but dropping the yo sounds much more natural in everyday speech unless you specifically want to emphasize that you are the one who does not know.

I learned another verb for "to know" (conocer). Why are we using saber here?

Spanish has two common verbs for "to know". We use saber for knowing facts, pieces of information, or how to do something. Because an answer is a piece of information, we use saber. The verb conocer is used for being familiar with people, places, or things, like knowing a person or having visited a city.