"Ma" becomes "mon" with feminine nouns starting with a vowel or mute h (French Possessive Adjectives)

The use of the possessive adjectives with feminine nouns is different in French and in English.

How to use possessive adjectives with feminine nouns in French

In most cases, you use the possessive adjectives ma, ta, or sa with a feminine noun (ma valise, ma petite sœur). See Mon, ma, mes; ton, ta, tes; son, sa, ses = my; your; his / her in French (French possessive adjectives)

HOWEVER, you need to know two things:

1. The possessive adjective's ending is always determined by the word directly following it

2. You never use ma, ta, sa in front of a word starting with a vowel or mute h, even if the related noun is feminine: you use mon, ton, son instead, to ease pronunciation. 

Here are the different contexts where this rule matters:

 

mon / ton / son + feminine noun starting with a vowel/mute h

mon ami Thomas et mon amie Sarahmy friend Thomas and my friend Sarah

Mon aventure est une aventure passionnante.My adventure is an exciting adventure.

J'ai une horloge qui est plus grande que ton horloge.I have a clock which is bigger than your clock.

Son opinion est une opinion intéressante.His opinion is an interesting opinion.

When ma, ta, sa is followed directly by a feminine noun starting with a vowel or mute h, it becomes mon, ton, son.

ma / ta / sa + adjective starting with a consonant + feminine noun starting with a vowel/mute h

C'est ton amie ? - Non, c'est ma petite amie.She is your friend? - No, she is my girlfriend.

Il se souvient de son enfance, sa belle enfance.He remembers his childhood, his beautiful childhood.

C'est mon hypothèse. - Oui, ta mauvaise hypothèse !That's my hypothesis. - Yes, your bad hypothesis!

If ma, ta, sa is followed directly by an adjective starting with a consonant, even if the related feminine noun starts with a vowel or mute h, it remains ma, ta, sa.

mon / ton / son + adjective starting with a vowel/mute h + feminine noun

J'ai goûté ton excellente tarte aux pommes !I tried your excellent apple tart!

Tu as rencontré mon horrible tante ?Have you met my horrible aunt?

Tu as vu son énorme verrue?Did you see her huge wart?

If ma, ta, sa is followed directly by an adjective starting with a vowel or mute h, even if the related feminine noun starts with a consonant, it becomes mon, ton, son.

The question of "y" 

In French, the letter "y" is (rarely) used as a vowelYves, cygne, gym... However, most words starting in "y" in French are of foreign origin, where "y" is used as a consonant: yacht, yaourt...
So here's the rule of agreement of mon/ma in front of feminine nouns starting with this peculiar letter:
- If the first letter "y" is used as a consonant, then it remains ma, ta, sa.
Ma grand-mère sort sa yaourtière.My grandma is taking her yoghurt-maker out.

Ma Youtubeuse préférée est Natoo.My favourite (female) Youtuber is Natoo.

- If the first letter "y" is used as a vowel (very rare), then it becomes mon, ton, son.
Il aimait son Yvette chérie.He loved his darling Yvette.

Je ne vois pas mon Yvonne ! Où est-elle ?I can't see my Yvonne! Where is she?

 
More Possessive Adjectives

Notre/nos/votre/vos/leur/leurs = our/your/their (French Possessive Adjectives)

and also the more advanced

Common mistakes with mon/ma/mes, ton/ta/tes and son/sa/ses (French Possessive Adjectives)

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Examples and resources

C'est mon hypothèse. - Oui, ta mauvaise hypothèse !That's my hypothesis. - Yes, your bad hypothesis!
C'est ton amie ? - Non, c'est ma petite amie.She is your friend? - No, she is my girlfriend.
une assiette -> mon assiettea plate -> my plate
Il aimait son Yvette chérie.He loved his darling Yvette.
mon ami Thomas et mon amie Sarahmy friend Thomas and my friend Sarah
Son opinion est une opinion intéressante.His opinion is an interesting opinion.
Je ne vois pas mon Yvonne ! Où est-elle ?I can't see my Yvonne! Where is she?
Mon aventure est une aventure passionnante.My adventure is an exciting adventure.
Ma grand-mère sort sa yaourtière.My grandma is taking her yoghurt-maker out.
J'ai une horloge qui est plus grande que ton horloge.I have a clock which is bigger than your clock.
Tu as rencontré mon horrible tante ?Have you met my horrible aunt?
Ma Youtubeuse préférée est Natoo.My favourite (female) Youtuber is Natoo.
J'ai goûté ton excellente tarte aux pommes !I tried your excellent apple tart!
Tu as vu son énorme verrue?Did you see her huge wart?
Il se souvient de son enfance, sa belle enfance.He remembers his childhood, his beautiful childhood.
Thinking...