Do I use capital letters when writing Mon ami?

Yes, you should use capital letters when writing "Mon ami" in French.

Here's why:

* "Mon" is a possessive adjective: It means "my" and functions similarly to "my" in English. Possessive adjectives are always capitalized in French.

* "Ami" is a noun: It means "friend." Nouns are also always capitalized in French.

Therefore, the correct way to write "Mon ami" is with capital letters: Mon Ami.