"You know my wife."
Translation:Vous connaissez ma femme.
"mon" is used in 2 cases:
- if the object owned is masculine and singular, like "mon mari" = my husband
- if the object owned is feminine but starts with a vowel sound, like "mon épouse" = my wife
In essence, possessive adjectives (mon, ma, mes...) agree in gender and number with the object possessed not with the owner.
"une épouse" is a wife in a higher register of speech than "une femme" and the masculine is "un époux".
You probably used a masculine possessive, which you should not since "mon, ma, mes" and all other possessives are adjectives and agree with the noun they modify: "ma femme" is correct.
The system suggested "épouse", which is obviously feminine, after your "mon", because "épouse" starts with a vowel and in this case, "mon" replaces "ma" to avoid the vowel sound conflict.