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When do we use たくさん and when do we use 多い? I found both modifiers are used
within similar sentences, for example:

> 人が多かったです  
>  人がたくさんいました

I had this impression that 多い is used with countable nouns (i.e. similar to
"many") while たくさん can be used with both countable and uncountable, but seems
like it is used with "お金":

> [ふだんの生活で使う **お金が多い**
> からか――ということです](http://doraku.asahi.com/money/moneygaku/091023_02.html)

At first, I thought maybe in Japanese language "お金" is considered as
countable, unlike the word "money" in English, but again I realized I was
wrong when I found:

> [きょうは **水が多い**
> ですね](http://mytown.asahi.com/shizuoka/news.php?k_id=23000381104180001)

Now I'm thinking that maybe the concepts of countable vs uncountable in
Japanese are not as significant as in English. It seems to me that both たくさん
and 多い may be used interchangeably.

Am I right? Or are there actually places where only one of them can be used
but not the other?

EDIT:

Additional question: how are they related to potentially antonyms 少し and 少ない?
Is 少し antonym of たくさん while 少ない is antonym of 多い, or vice versa, or it does
not matter either way?
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question
word-choice usage
Usage of たくさん vs. 多い
In response to the found statement:

```

    "今日は水が多いですね"
    
```

This does not make sense. Literally, "There is a lot of water today." So since
it is not sensible, i would not use it to debunk your thoughts.

Let's consider rain and the two following statements to try and express "It
rained a lot today, didn't it?"

```

    今日は雨がたくさん降りましたね。
    今日は雨が多く降りましたね。
    
```

Simply, the second is nonsensical. Here you can see the clear difference
between 多い being countable and たくさん as either countable (as shown in your
question) or uncountable. Instead, what you could say to use 多い and rain would
be: (possibly what the initial 水が多い statement was meaning.

`最近雨が多いです`。The frequency was high.

Also, another thing to take note of for those who are newish to this kind of
grammar is that たくさん is an adverb and 多い is an adjective and so the normal
rules apply. adverb + verb, adjective + be-verb (です) or noun.

Now, to clarify, I am quoting from the Japan Times "A Dictionary of Basic
Japanese Grammar", pg 354. This also slightly contradicts my answer above.

多い means (of quantity or number) a lot, many, a lot of, much. Similar to
大勢、たくさん. (大勢, it is noted, is only used for people.)

Going of this and other articles, like the one from Asahi, 多い is used
correctly when referring to the quantity of an uncountable material or thing
(like snow, water, rain) but is not as flexible as たくさん. I will illustrate
with a few samples from the text.

```

    1. 京都（に）はお寺が多いです。
    2. ロスさんの作文（に）は間違いが多いです。
    3. 一月（に）は雪が多い。
    
```

"Unlike the English 'many', the Japanese 多い cannot be used before a noun,
except in a relative clause where 多い is the predicate of the clause, not the
modifier of the head noun." This is an example where 多い is not as flexible as
たくさん.

```

    4. 京都（に）は多いお寺があります。　Incorrect. Corrected is 京都には多くのお寺があります。  
    5. お寺が多い町は京都です。 Correct. (the predicate of a clause thingy)
    
```

There are also examples of where たくさん can and cannot be used in place of 多い.

```

    6. この部屋（に）は つくえが多い。/たくさんある。  
    7. この部屋（に）は学生が多い。/たくさんいる。  
    8. 今年はたくさん/*多く/*大勢雪が降った。  
    
```

多く can be used as a noun, but the other two cannot.

```

    9. 学生の多く/*大勢/*たくさんは男性です。
    
```

(* denotes incorrect usage)

So in a nice long winded half incorrect answer, we can see that 多い is not
strictly used for countable objects, nor can たくさん simply be swapped with 多い・多く
in any situation. Just follow the simple grammar rules, now knowing these
extra couple of things, and you should have no problem with using them, I
think.
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answer
Here's a quick translation of the [page that
repecmps](http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1325928873)
linked to

多い is an adjective, while たくさん is a noun which is also used as an adverb, but
there are differences in their uses. There are also some differences in their
meaning. 多い and たくさん both count things, number of times, quantities, etc but
for strictly measuring things, 多い is used.

今年の交通事故者数は昨年より１００名多い。 - Compared to last year there were 100 more traffic
accidents.  
今年の交通事故者数は昨年より１００名たくさんいる - This sentence is incorrect.

今日の練習はいつもより３回多い We did 3 times more practice than usual today.  
今日の練習はいつもより３回たくさんやった - This sentence is incorrect

While you can understand the meaning of the incorrect sentences, they sound
like a child said them.

In the following examples, you can use either:

その牧場には虫が多い  
その牧場には虫がたくさんいる  
There are many insects in that farm

夏はかく汗の量が多い  
夏は汗をたくさんかく  
In summer, you sweat a lot.

The following are special cases where you can only use one or the other.

300ｃｃは多いよ - 300cc is too much  
カレーライスはもうたくさんだ - I've eaten enough curry (I can't eat any more).

You can see here that 多い is used with measurements while たくさん is used with
emotions.

[If you spot any mistakes in this translation, feel free to edit this answer]
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answer
I don't think the existing answers are hitting this question from quite the
right angle, so here is my take:

First, in sentences where you only wish to mention the presence of "a large
number" or "a large quantity", 多い and たくさん can often be interchanged, provided
you do some grammatical fiddling to get the rest of the sentence to work:

> 人が **多い** ですね。 There are **a lot of** people here, huh?
>
> 人が **たくさん** いますね。 There are **a lot of** people here, huh?
>
> **多くの** 人が参加しました。 **Many** people participated.
>
> **たくさんの** 人が参加しました。 **Many** people participated.

But there's an alternate interpretation for the third sentence: 多くの could
simply mean "a lot", but it could also mean "a large proportion". It's this
idea of largeness in proportion or ratio that たくさん does not have. This shows
up in sentences comparing one quantity to another:

> 今日は水が **多い** ですね。 There's **more** water **[than before]** today.
>
> これからは機会が **多く** なりそうです。 It looks like we'll have **more** opportunities
> **[than before]** from now on.
>
> 今年の春は去年の同じ時期より雨が **多く** 降りました。 This spring, **more** rain fell than during
> the same time last year.

Note that 雨が多く降った is a legal sentence, but only within the proper context of
comparing proportions, as demonstrated by the third sentence above.

By extension, 多い comes with a meaning similar to "more than necessary". This
is another meaning たくさん does not have:

> 10人ならいいけど、20人はちょっと多いですね。 10 people would be good, but 20 is a bit much.
>
> あのさ、これ、多くない？ Hey, isn't this too much?

You cannot replace 多い with たくさん in the above examples, because たくさん lacks the
ability to compare a quantity to a certain level.

In response to your additional question about 少し and 少ない: Yes, you can think
of these two as corresponding with たくさん and 多い, respectively. 少し only refers
to quantity, but 少ない has the additional meanings of "small in proportion" and
"less than necessary".
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answer
As you suspected, countability is not significant in Japanese. The difference
has nothing to do with it. The difference is that `多い` is an adjective while
`たくさん` is used adverbially. `多い` can be translated to English as `many`,
`vast`, etc. The English counterpart to `たくさん` may be `massively`, `to a large
extent`, etc. So, literal translation of your examples will be:

```

    人が多かったです
    `The (amount of) people was/were vast'
    
    人がたくさんいました
    `There were people to a large extent'
    
```

Interchanging them will be ungrammatical or unnatural just as the
corresponding Engligh sentences are:

```

    人がたくさんです
    `The (amount of) people was/were to a large extent'
    
    人が多くいました
    `There were vast people'
    
```
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answer