Why is 'Futatsu' instead of 'Ni'?
つくえが二つあります 'there are two desks'
I know kanjis use to have two sounds at least, I mean why in this case instead of using 'ni' alone, it also has a 'tsu'. I'll thank if you could explain me why. By the way, sorry if I didn't write correctly, my english is not good at all.
ni and futa are two possible readings for 二, whereas the つ is called a classifier added when counting, because things in Japanese are counted by adding this classifier that depends on the nature (size, shape, etc.) of what’s being counted
In English we say: one apple, two apples.
In Japanese we say りんご 一個、二個 because 個 (ko) is the classifier used to count apples (small in size but not thin or long).
In English : one CD, two CDs.
In Japanese: シーディー 一枚、二枚 because we use 枚 (mai) , the classifier used for counting flat objects.
And so on
You’re welcome. Actually the topic of classifiers is important and marks one of the fundamental differences between Japanese and English, and I wish the course had a special skill dedicated to it. If it helps, this Wikipedia page introduces some of the most common classifiers in Japanese.
There is a native Japanese numeral system (used from 1 to 10 only), and there is the Sino-Japanese numeral system. に（二） is part of the Sino-Japanese system.
つ is a generic counter for things. Other examples of counters are まい（枚） for thin objects like paper, and ほん（本） for long objects like pencils.
The native Japanese counters from 1 to 10 are:
ひとつ（一つ）, ふたつ（二つ）, みっつ（三つ）, よっつ（四つ）, いつつ（五つ）, むっつ（六つ）, ななつ（七つ）, やっつ（八つ）, ここのつ（九つ）, とお（十）.