Difference between один / одна / одну, два / две in survival kit lesson - Numbers, Decimals and Dollars

Здравствуйте!
there is something else that I don't understand in the lesson Numbers, Decimals and Dollars (survival kit) in the following examples:

одна целая пять десятых
Я хочу одну

Why is it not один целая пять десятых or Я хочу один?

Same question about два in these two examples:

Я хочу две
Мне семьдесят два года

How can I know when I should use either две or два?

Спасибо! Thank you

Здравствуйте Matthieu13!

It's true Russian numbers can be tricky, particularly один and два.

The word один changes to match the grammatical form of the following word, e.g.:

один год (one year) - masculine
одна минута (one minute) - feminine
одно пиво (one beer) - neuter
It also has other case forms.

The word "two" has only two forms: два and две.
Два is used with masculine and neuter nouns:
два года (two years)
два пива (two beers)
Две is used with feminine nouns:
две булки (two loaves)
две сестры (two sisters)




 

Спасибо, Natalya, I understand now!