日本語の使用語彙（自分が使える単語）数は、
１才で１０個
６才で２５００個
１０才で１００００個
と言われています。
自分は使わないけど理解できる単語数はもっと多く、１０才だと３００００語と言われています。

語彙数が多いほど、日本語の理解力も当然高くなります。
日本語教室では、日常会話では使わない学習単語などをていねいに個別に教えることで、授業の理解力や語彙力アップを行っています。

The number of vocabulary used in Japanese (words that you can use) is
1 year-old →10
6 year-old →2500
10 year-old →10000
It's said that there are more than 30000 words at the age of 10 which they just understand and don't use.

The more vocabulary you have, the better your understanding of Japanese will be :)
In the Japanese language class, I'm trying to improve the comprehension and vocabulary of the lesson by carefully and individually teaching learning words that are not used in daily conversation.