nhiều

1. With Nouns (nhiều + Noun)
Place nhiều before a noun to mean 'many' or 'much'. It is frequently paired with rất to mean 'very many' or 'a lot' (rất nhiều).

• nhiều người = many people
• rất nhiều tiền = a lot of money

Note: Unlike in English, nhiều cannot stand alone as a subject to mean 'many people' or 'many things' (e.g., you cannot say 'Many believe...', you must say Nhiều người tin rằng...). However, it can act as a subject when referring to 'a large amount' or 'too much' (e.g., Nhiều quá không tốt = Too much is not good).

Optional Classifiers:
When using nhiều with countable nouns, the classifier is optional. Both nhiều chó and nhiều con chó (many dogs) are perfectly natural.

2. As a Predicate (Noun + nhiều)
Because Vietnamese adjectives work like verbs, nhiều can act as the main predicate to mean 'there is a lot of [noun]' or '[noun] is abundant'. It is often paired with degree words like quá or lắm.

• Bài tập nhiều quá! = There is so much homework!
• Lỗi nhiều lắm. = There are a lot of mistakes.

3. With Verbs (Verb + nhiều)
To say you do something 'a lot', place nhiều after the verb. If there is an object, the placement of nhiều changes the meaning:

• Verb + nhiều + Noun: Focuses on the quantity of the noun. (Tôi ăn nhiều cơm. = I eat a lot of rice.)
• Verb + Noun + nhiều: Focuses on the frequency or amount of the action. (Tôi ăn cơm nhiều. = I eat rice a lot.)

4. More vs. Much More (nhiều hơn vs. hơn nhiều)
Be careful with word order when making a comparison:

• nhiều hơn means 'more' (a larger quantity). For verbs, place it after the verb (Tôi muốn ăn nhiều hơn. = I want to eat more). For nouns, it usually splits around the noun: nhiều + Noun + hơn (Tôi cần nhiều thời gian hơn. = I need more time). Note that to express an additional amount rather than a comparison, you should use thêm instead of hơn.
• hơn nhiều means 'much more' (a larger degree). Place it after an adjective (tốt hơn nhiều = much better / to hơn nhiều = much bigger). It is frequently intensified with rất (e.g., tốt hơn rất nhiều = very much better).

5. The Opposite: ít (Few / Little)
You can often swap nhiều for ít to mean 'few', 'little', or 'less'. However, be careful with comparisons: you cannot swap nhiều for ít to say 'a little better' (tốt hơn ít is unnatural and incorrect). To say 'a little better', you must use tốt hơn một chút or tốt hơn một ít. To say 'much less [adjective]', you must use ít [adjective] hơn nhiều or kém [adjective] hơn nhiều.

6. Asking 'How Much / How Many'
To ask questions about quantity, use the counterpart to nhiều, which is bao nhiêu (how much / how many).

7. With Time Words (nhiều khi, nhiều lúc)
When combined with time words like khi or lúc, nhiều forms adverbs of frequency meaning 'often' or 'sometimes' (e.g., nhiều khi, nhiều lúc).

8. Reduplication (nhiều nhiều)
In spoken Vietnamese, you might hear the reduplicated form nhiều nhiều. It is often used to mean 'a little bit more' or 'quite a lot' (e.g., Cho tôi nhiều nhiều một chút = Give me a little bit more).