Learn Spanish (Spain): Sé que la experiencia de mi abuelo es peor. - I know that my grandfather's experience is worse.

Peor (worse) is an irregular comparative adjective. Just like in English we say 'worse' instead of 'more bad', Spanish uses peor instead of más malo or más mala.

La experiencia is feminine. Does peor change to match its gender?

No, irregular comparatives like peor (worse) and mejor (better) only change for number, not gender. You use peor for both masculine and feminine singular nouns. If the noun were plural (worse experiences), you would add an -es to make it peores.

Why does Sé (I know) have an accent mark?

The accent mark tells us that this is the verb saber (to know). It is used to distinguish it from se, which is a pronoun used without an accent mark (like in él se llama, meaning 'he calls himself').

Why use sé instead of conozco for 'I know'?

You use saber (which becomes sé for 'I') when you know facts, information, or how to do something. Here, you know a fact: that the experience is worse. You would use conocer (conozco) if you were talking about being personally familiar with a person, place, or thing.

Why do we say instead of using an apostrophe 's' like in English?