Excess and sufficiency: too big, enough time
Before an adjective or adverb, demasiado means "too" and never changes.
Este piso es demasiado caro. (This flat is too expensive.)
Hablas demasiado rápido. (You speak too fast.)
Before a noun it agrees: demasiado, demasiada, demasiados, demasiadas.
Hay demasiado ruido. (There is too much noise.)
Tengo demasiadas cosas que hacer. (I have too many things to do.)
Suficiente(s) = enough. It usually goes before the noun and only changes for number.
No tengo suficiente dinero. (I do not have enough money.)
¿Hay suficientes sillas? (Are there enough chairs?)
"Adjective + enough" (big enough) = lo suficientemente + adjective. Spanish flips the English order.
No es lo suficientemente grande. (It is not big enough.)
Eres lo suficientemente mayor para entenderlo. (You are old enough to understand it.)
A simpler everyday alternative: bastante — Es bastante grande. (It is big enough / quite big.)
After a verb, demasiado = "too much" and never changes.
Trabajas demasiado. (You work too much.)
Comí demasiado. (I ate too much.)
Traps for English speakers
These are the errors English speakers make most often.