Numbers from 0 to 100 are the easiest to master. However, there are a few irregularities, especially certain differences between units, teens and tens. Moreover, the term pluralia tantum identifies nouns which exist solely in their plural form.
Number 1
At first, it may look strange that something as simple as number one gets is own category. However, this number is gender sensitive and changes depending on what it describ...
Here are some lessons you can try out straight away:
