Trying to understand numbers


I started learning Japanese not long ago and I'm quite confused about two
numbers.

Seven and nine, I have learned that seven is shichi and that nine is ku.  
But I also know that seven is nana, and nine is kyu, actually I cannot
remember ever hearing anyone use shichi and ku.

So basically my questions are, why there is more then one word for those
numbers in particular and when to use which form.




About pronunciations:

> Shichi/Nana and Kyuu/Ku and Shi/Yon

Both are all correct, this is a matter of taste, but in some expressions you
have to use one in particular.

Japanese will use either _shichi_ or _nana_.

People usually use _kyuu_ except in some words like _kugatsu_ as said by
Sqrtbottle.

Also Japanese would rather not use _shi_ as it has the same pronunciation as
the _shi_ in _Shinu_ which means death, but both are perfectly correct and are
a matter of taste. I know several Japanese who use _shi_.

About the meaning of numbers:

It is wrong to say the Chinese root ones are more commonly used. This is just
a different usage. Basically if you start learning Japanese you should start
by learning:

> いち に さん し/よん ご...

Those are mostly used for mathematics and calculations.

But for speech, there are a lot of different counting systems. (to count
things) The most basic one is like this:

> ひとつ ふたつ みっつ よっつ ...

You are supposed to use it for counting things, but this is not as simple as
that. If you start learning, only knowing both is ok. But to understand daily
Japanese you need to learn a lot of different counters:

> いっぽん にほん さんぼん (counter for long stuff like bottles, straws etc ...)  
>  いちまい にまい さんまい (counter for thin stuff like sheets of paper)  
>  いっぱい にはい さんばい (counter for cups of drink)  
>

There are a lot of those.

So basically, they all have their own usage. If you don't know the special
counters it's ok, just use the basic version (ひとつ ふたつ...), your grammar will
be correct. But using special counters is always better, it lets you build
shorter sentences and will sound more Japanese.

Don't forget, you are not supposed to count stuff with いち に さん.

Let's do some examples:

> いちたすいち : 1 plus 1
>
> いっぱいちょうだい : give me one cup

As you can see in the second one, all you can see is the counter for cups and
_choudai_ , so implicitly you understand that we are talking about cups. One
last example:

３まいたりない : literally means "I lack 3 thin objects", but depending on the
context, you could be printing things in the office, people will understand it
like: "I need 3 more sheets of paper to finish my print", this is how this
sentence would be translated and how Japanese will understand it. Saying
[紙]{かみ}がみっつ[足]{た}りない might be grammatically correct but it doesn't sound good.




The forms derive from two roots: Japanese ones and Chinese ones.

Japanese ones are 1 ひ、2 ふ、3 み、4 よ(ん)、5 い(つ)、6 む、7 _なな_ 、8 や、9 ここの、10 とお etc.  
Chinese ones are 1 いち、2 に、3 さん、4 し、5 ご、6 ろく、7 しち、8 はち、9 きゅう (also く)、10 じゅう

Generally the Chinese ones are more common. They're used for counting objects
with counters (if you know what these are), like animals and objects, and even
just numbers on their own (like 1 + 1 = 2).

The Japanese ones are used in counting things with a ~つ suffix, which is the
general counter for objects, and you also see them in some words like ひとり (one
person) and ふたり (2 people), as well as in some of the days of the month, and
in peoples' names.

_Which is more common?_

Japanese ones see much more limited usage, but people will use よん in place of
し and なな in place of しち to make it clearer which they mean (as they sound
similar). These two are mixed in with Chinese numbers in usage quite a lot.

Which to use needs to be done on more of a case-by-case basis depending on
what you're talking about, but on the whole Chinese numbers (ichi, ni, san)
are more commonly used.

**ku**

Not that frequently used, and never on its own. You'll usually hear this one
(irregularly) instead of きゅう when counting things (September is "kugatsu"
rather than "kyuugatsu").

