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Of course I can use these but sometimes I am corrected by Japanese people when I use them in the wrong circumstance.

For example,

  • 「よくしってるやん」 = "You know it quite well don't you!"
  • 「よくいきます」= "I go (there) a lot"
  • 「皿がたくさんあります」= there are a lot of plates       
  • 「人がいっぱいいる」= there are lots of people

Just wondering if anybody has any concrete rules that I can follow so that I don't make any (or as many) mistakes in the future.     

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This is not the main point of the question, but if I sometimes use them in wrong circumstances, I would not say “of course I can use these.” – Tsuyoshi Ito Jun 16 '12 at 23:16

「[沢山]{たくさん}」 and 「[一杯]{いっぱい}」 both refer to the noun, but the former means "many" so it would be used for an arbitrarily large quantity, whereas the latter means "full" so it would be used more to refer to the capacity of whatever is containing the quantity.

「[良]{よ}く」 is the adverb form of 「[良]{い}い」, and refers to the verb, to indicate an "augmented state" appropriate for the verb.

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thank you very much! – Seb Jun 15 '12 at 8:25
Just out of curiosity, could 一杯 be used for expressing "full"ness of things that don't have physical volumes? (e.g. emotions and the like) – Miguel Jun 15 '12 at 17:23
@Miguel: An interesting question, but not one that I know the answer to. Perhaps it's worth asking separately. – Ignacio Vazquez-Abrams Jun 15 '12 at 17:28
Done... :) – Miguel Jun 15 '12 at 17:57

たくさん means that the quantity is large: “many,” “much,” and “a lot.” As Ignacio Vazquez-Abrams said, いっぱい means “full,” but いっぱい is colloquially also used to mean the same thing as たくさん. (In child-speak, they also mean that the degree is high: 今日は(いっぱい/たくさん)遊んだね (you played a lot today).)

Adverb よく does not mean that the quantity is large. It means:

  • very much, when talking about degree (not quantity): よく知ってるやん is an example of this.
  • often: よく行きます is an example of this.

よく is also the continuative form of adjective よい/いい.

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