Mandarin Learning Basic

If in English, we just use a, i, u, e, o and the combination for vocal. But in Mandarin there is some character can’t be represented by them, like女. This character not spell in ‘ni’ or ‘nu’ but ‘nuu(3)’ (I found from some mandarin language software), how can to spell it right? I don’t know how to explain it. This vocal is between in ‘i’ and ‘u’, later maybe I will upload some audio to explain it.

The 女 means ‘female’ can be used in various kind of combination word. Ok, let’s see the sample below:
老处女 spelling ‘lao(3) chu(4) nuu(3)’ that’s have mean ‘old maid, spinster’
女人 spelling ‘nuu(3) ren(2)’ that’s have mean ‘woman’
女孩 spelling ‘nuu(3) hai(2)’ that’s have mean ‘girl’
美女 spelling ‘mei(2) nuu(3)’ that’s have mean ‘beauty girl’

Sometimes people feel difficult to learn Mandarin / Chinese language. But myself, it’s more difficult to learn English, so it’s a way I can improve my English skill too..and I am sorry if my English is not good.