[Sumippi]

おお！I didn't know about the 'x'-key.

ゃ-- lya / xya
ゅ-- lyu / xyu
ょ-- lyo / xyo
ぉ-- lo / xo
ぇ-- le / xe
ぅ-- lu / xu
ぃ-- li / xi
ぁ-- la / xa
ゎ-- lwa / xwa

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＞And for number 20, can I say?　juujuu
＞20 = 10 + 10 = juu + juu = juujuu

--No, 20 is にじゅう(ni-juu). And 20th (for the day and duration) is はつか(hatsuka). We understand にじゅうにち(nijuu-nichi), but I think we usually say はつか.

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＞I currently learned the numbers, can you check them, please?
Twenty-One: 2(10) + 1 = ni-juu-ichi
Twenty-Two: 2(10) + 2 = ni-juu-ni
Twenty-Three: 2(10) + 3 = ni-juu-san
Twenty-Four: 2(10) + 4 = ni-juu-shi
Twenty-Five: 2(10) + 5 = ni-juu-go
Twenty-Six: 2(10) + 6 = ni-juu-roku
Twenty-Seven: 2(10) + 7 = ni-juu-shichi
Twenty-Eight: 2(10) + 8 = ni-juu-hachi
Twenty-Nine: 2(10) + 9 = ni-juu-ku
Thirty: 3(10) = san-juu
Thirty-One: 3(10) + 1 = san-juu-ichi
etc....

--All correct.^^

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＞Is it September Sixth?
＞kugatsu muika desu ka.

--The subject 「きょうは=today」is left out here. You can also say 「きょうは、くがつむいかですか」.

You may think 「きょうは、くがつむいかですか。」(Kyou wa, kugatsu muika desu ka. ) would be grammatically correct, but we often say this without 'Kyou wa',-- because it can be implied, I think.... It depends on the context, though.