Quantity words that agree (and the ones that refuse to)
Where English splits much/many, Spanish has one word — mucho — that agrees with the noun in gender and number.
mucho tiempo (much time) · mucha gente (many people)
muchos libros (many books) · muchas veces (many times)
Four forms: mucho, mucha, muchos, muchas. The same goes for poco (little/few) and demasiado (too much/too many).
Poco = little/few; demasiado = too much/too many. Both agree like mucho.
Tengo poco dinero y pocas vacaciones. (I have little money and few holidays.)
Hay demasiada gente y demasiados coches. (There are too many people and too many cars.)
Note gente is singular feminine in Spanish — mucha gente, demasiada gente — even though English "people" is plural.
After a verb or before an adjective, mucho/poco/demasiado are adverbs — and adverbs never agree.
Ella trabaja mucho. (She works a lot — never "mucha".)
La sopa está demasiado caliente. (The soup is too hot — demasiado + adjective, invariable.)
Test: is there a noun right after? Agree. Is it after a verb or before an adjective? Invariable.
Bastante means "quite a lot / enough"; suficiente means strictly "sufficient". They only change for number (no gender forms).
Hay bastantes sillas. (There are enough/quite a few chairs.)
No tenemos suficiente tiempo. (We don't have enough time.)
El examen fue bastante difícil. (The exam was quite hard — adverb, invariable.)
Bastante before an adjective = "quite": bastante caro = quite expensive.
Alguno (some) and ninguno (none) drop the -o before a masculine singular noun: algún, ningún.
¿Tienes algún problema? (Do you have any problem?)
No tengo ningún problema. (I have no problem.)
No queda ninguna entrada. (There are no tickets left.)
Like the double negative with nunca: No tengo ningún problema — the two negatives are required, not wrong.
Traps for English speakers
These are the errors English speakers make most often.