How do I recognize a vowel? Because V in vingt is not a vowel yet je is replaced with j' in the sentence j'ai vingt ans

Salut Esther, a vowel is a, e, i, o, u and for these purposes a silent h. However, in your example the verb is avoir which follows je. So, je + ai = j'ai. The first letter of the word following the verb makes no difference.

Hope this helps.

Correct. V in Vingt is NOT a vowel. Je isn't placed as j' because of "vingt" but because "J'ai" is how you normally say "I have". It's as normal as saying "Tu as" or "Nous avons". Hope this helped!

why not put a direct object pronoun in a sentence eg for do salle de bain