I'll give it a shot.
1. When "
tan"is used as an adverb modifying another adverb or an adjective, it means "
so".
Ella es tan metódica. = She is so methodical.
Es tan grande que no me lo puedo imaginar. = It is so big I can't fathom it.
Sucedió tan rápidamente... = It happened so quickly...
2. Sentences using
tan often can be translated using the word
such in English.
Tiene una barba tan grande. = He has such a big beard.
Está tan lejos. = It's such a long way away (It's so far away).
3.
Tanto (and
tantos, tanta, tantas). These are adjectives, used to modify nouns, and are ususlly translated "
so much" or "
so many". Tanto must agree in number and gender with the noun it modifies, so in many cases it must change to
tantos, tanta or
tantas.
¡Tengo tanto trabajo! = I have so much work.
Traigo tantos libros que no aguanto mi mochila. = I'm bringing so many books I can hardly carry my backpack.
He visto tanta televisión que me duele la cabeza. = I've watched so much TV that my head hurts.
¡Tengo tantas ganas de ir! = I want to go so much!
4. Comparisons (tan x como y = as x as y).
El no es tan viejo como yo. = He is not as old as I am.
No tengo tanto dinero como ella. = I don't have as much money as she does.
Vi tantos coches como tú. = I saw as many cars as you did.
Tomo tanta agua como tú. = I drink as much water as you.
Tengo tantas canicas como tú. = I have as many marbles as you.
With these comparisons, what's the difference between tan and tanto/tantos/tanta/tantas? It's simply this: If the word you're modifying is an adverb or adjective, you need to use tan. If the word you're modifying is a noun, use tanto, and make sure it agrees in number and gender with the noun (tanto/tantos/tanta/tantas). Look at the above examples. Viejo is an adjective, while dinero, coches, agua and canicas are nouns.