Learn Italian: È un problema mio, non tuo. - It is my problem, not yours.

Placing the possessive adjective after the noun adds strong emphasis. While un mio problema simply means a problem of mine, un problema mio stresses that the burden belongs specifically to me. This word order sets up perfectly for the direct contrast with non tuo at the end of the sentence.

Why doesn't tuo have an article before it, such as non il tuo?

When a possessive pronoun is used in a direct, short contrast like this or when it follows the verb to be, the definite article is usually dropped. You could technically say non il tuo, but omitting the article makes the contrast much punchier and sounds more natural to a native speaker.

Problema ends in -a, so why do we use the masculine modifiers un, mio, and tuo?

Nouns of Greek origin ending in -ma are almost always masculine in Italian. While you likely learned this rule early on, it remains a very common slip of the tongue for B2 learners to treat them as feminine when speaking quickly. Sentences like this are great anchors to help permanently fix the masculine gender in your mind.