Tan is an adverb and it is used with adjectives;
\nTanto is an adjective and it is used with nouns.
\nEnglish: \u2026as fast as\u2026Espa\xf1ol: \u2026tan r\xe1pido como\u2026
\nEnglish: \u2026as big as\u2026Espa\xf1ol: \u2026tan grande como\u2026