Quick translation question

For the number seven in Japanese, is either "shichi" or "nana" acceptable? What is the difference? Should both be taught? Thank you.

Both shichi and nana mean 7, just like both shi and yon mean 4, and like ku and kyuu mean 9. You should learn both versions of all these numbers.

The difference in use is about environment. So for example, July (the seventh month) is shichi-gatsu, but 7 years old is nana-sai. You would never say nana-gatsu or shichi-sai. As far as I know, though, there's no system to decide which reading to use, so you just have to memorize it, but you'll get the hang of it after awhile.

I'm confused about nine now -- I thought it was:

nine kyu (pronounced kyoo)

?

I'm finding different ways to spell 10 -- isn't it:
ten jyu (pronounced joo)

?

Which numbers would you use:

yon or shi -- to count four castles

shichi or nana -- to count seven of the ducks?

Or can you use either?
Thanks!

9 can be ku or kyuu. It might help pronunciation to learn romaji.

(all examples should be pronounced as they are in "5 o'clock news" English)
a- all
i- eat
u- uzi
e- exit
o- open

The consonants are mostly the same, except for h/f which is more variable.

10 is jyuu or juu depending on if you're using the older or newer system of romanization. You have to remember that Japanese doesn't use roman characters (the ABCs) so whether or not you have the spelling right is not so important as all that. Learning to read hiragana should clear up pronunciation issues entirely.

As for the question about counting things, a whole different system comes into play. Japanese uses "counters" to classify things based on shape and they are tacked on to the end of a counting system that is not always based on ichi, ni, san, etc.. Like how in English you can't say "Give me one bread," but rather "Give me one slice of bread," or "one loaf of bread."

castles are buildings, so the counter is -ken. Three castles is "san-ken no shiro". Ducks are (small) animals, so the counter is hiki/biki/piki. Seven ducks is "nana-hiki no kamo".

If you don't want to memorize the (huge amounts of troublesome) counters all at once, I would suggest learning both of the the "default" counters, -ko (round objects/boxes/things shaped like that) or -tsu (multipurpose or otherwise unspecified shape)

-ko is similar to the ichi, ni, san that you're used to, with some modifications, -tsu is completely different:

1 ikko hitotsu
2 niko futatsu
3 sanko mittsu
4 yonko yottsu
5 goko itsutsu
6 rokko mutsu
7 nanako nanatsu
8 hakko yattsu
9 kyuko kokonotsu
10 jyukko to

tsu is especially hard to memorize, but if you learn it it will help you with dates (August 8th, etc.) later.

couldn't remember how many castles.

It would be yon-ken.

When you use counters, you almost always use yon and nana, not shi and shichi.

I have not been able to find the right counter for castles - actually "-ken" would be more for regular houses! If you wanted to say something like "I've visited four castles," I think I would rephrase it to say "I visited castles at four locations." In a way every one is unique and would feel somewhat disrespectful to bunch them up to count lol.

Thus I would say:
- Nihon de yon-kasho o-shiro wo houmon shimashita. (In Japan I visited castles at four places/locations.)

About ducks, I might prefer to use "-wa" for birds. So "seven ducks" would be "nana-wa no kamo."

Japanese Numbers: 1-10
one ichi (pronounced ee-chee)

two ni (pronounced nee)

three san (pronounced sahn)

four yon (pronounced yohn) or shi (pronounced shee)

five go (pronounced goh)

six roku (pronounced roh-koo)

seven nana (pronounced nah-nah) or shichi (pronounced shee-chee)

eight hachi (pronounced hah-chee)

nine kyuu (pronounced kyoo)

ten jyuu (pronounced joo)

THANK YOU!