If it were "my robe" then it would be "ma robe", since it is "my robes" it is "mes robes". There is no difference at all in pronounciation between "robe" and "robes", but there is a noticeable difference between "ma" and "mes". Get used to it, since that's how a lot of stuff ends up working in French.
If you take a look at the Tips & Notes, you will get explanations about the French possessive adjectives.
Like all French adjectives, they agree with the noun they modify.
- "mon manteau" is masculine singular = my coat
- "ma robe" is feminine singular = my dress
- "mes manteaux et mes robes" is used for masculine or feminine plural nouns = my coats and my dresses.
Note that when a feminine singular noun starts with a vowel sound (vowel or mute H), "ma" is replaced with "mon" to avoid the vowel sound conflict:
- mon idée, mon orange, mon eau, mon huile: all these nouns are feminine and start with a vowel sound.
This helped sooo much, I am sometimes confused because of the plural differences, for some reason I just now relised that there is plural words like mes robes and ma robe! Thanks again!