Grammar JLPT Level N5 - Lesson 15

Structure of 沢山(たくさん):

たくさん + Phrase
たくさん +（の） + Noun

Details about 沢山(たくさん):

There are many cases in Japanese where a noun will appear as an adverb, particularly with words that are describing amounts. たくさん, or 沢山(たくさん) as it is regularly seen, is one of those words. たくさん means 'a lot', or 'many' in Japanese, and can be used before a phrase, or directly before a noun, when separated via の.

沢山(たくさん)食(た)べてよ!
Eat a lot! (Eat to your heart's content!)
この島(しま)には沢山(たくさん)の猫(ねこ)がいます。
On this island, there are a lot of cats. (Unrelated to the sentence, there is actually an island like this in Japan!)

沢山(たくさん)

Usage Notes for 沢山(たくさん):

たくさん may be used before a noun without also using の. However, in this case, it will feel more like たくさん is describing the whole phrase, rather than just the noun it is in front of. If you strongly want to highlight the noun, using の would be best.

あの店(みせ)の前(まえ)に沢山(たくさん)人(ひと)が集(あつ)まっている。
There are a lot of people gathering in front of that store. (Focuses on the store. I wonder why there are so many people in front of that store)
あの店(みせ)の前(まえ)に沢山(たくさん)の人(ひと)が集(あつ)まっている。
There are a lot of people gathering in front of that store. (Focuses on the people. The amount of people gathering up in front of that store is unbelievable)

In these examples, the only difference is that the sentence with の feels like the speaker is putting extra focus on the people, rather than the gathering that is happening.