The Spanish adverb 'tan' (meaning 'so' as in 'so big' or 'so beautiful') comes from the Spanish word 'tanto' (meaning 'so much'), which in turn derives from the Latin word 'tantus' meaning 'so much' or 'so great'. The Latin 'tantus' itself developed from an earlier Latin form 'tam' meaning 'so much, so'. Over time, the word was shortened from 'tanto' to 'tan' when used as an adverb in Spanish.

A closely related Spanish word is 'tanto' meaning 'so much' or 'so many'. While 'tan' is used as an adverb before adjectives (like 'tan grande' - 'so big'), 'tanto' is used as an adjective or pronoun (like 'tantos libros' - 'so many books'). Both words share the same Latin root and work together in the Spanish language to express degrees of intensity or quantity.

While there aren't any common English words directly derived from the same Latin root 'tantus' or 'tam', English speakers might find it helpful to think of 'tan' as similar in function to the English word 'so' when used as an intensifier. For example, 'tan bonito' (so pretty) works just like 'so' does in English.