People just learning Spanish often confuse the words mucho and muy. In fact, you can remember the difference pretty easily: just use mucho for “a lot” and muy for “very.”
Mucho – A Lot
The word mucho can be used as an adjective or an adverb.
Mucho as an adjective
Because an adjective modifies a noun, in Spanish, you have to conjugate the adjective to the noun: make it masculine or feminine, and singular or plural. You do this with mucho like with any other adjective: it ends with o/os for masculine nouns, and a/as for feminine nouns.
Mucho dinero A lot of money
Muchos libros A lot of books
Mucha suerte A lot of luck
Muchas mesas A lot of tables
Mucho as an adverb
Adverbs are words that modify verbs or adjectives. Because adverbs don’t directly modify a noun, you don’t have to conjugate them. So mucho as an adverb only has one form: mucho.
Mucho as an adverb can be used to modify verbs, but never adjectives. The phrase ‘a lot’ works the same in English: you can say, “he walks a lot,” but you can’t say, “he is a lot active,” for example. Note that when we use mucho as an adjective, it goes before the noun, like all adjectives. But when we use it as an adverb, it goes after the verb.
Tengo que trabajar mucho. I have to work a lot.
Ella estudia mucho. She has to study a lot.
Muy – Very
Muy is always an adverb that modifies an adjective. It never modifies a verb or noun. (Think about it in English: could you say very table? or very run? Nope–In English, too, we only use the word very to modify adjectives.) So, since muy is an adverb, it doesn’t get conjugated.
Carla es muy bonita. Carla is very pretty.
Ella es muy buena. She is very nice.