Sometimes, native people use grammar rules correctly without even knowing they’re grammar rules—they just come natural to them. To answer this question, I myself had to look it up in my old grammar books.
And the answer was easier than I thought:
In a grammatically correct Italian you always have to say “the” (il, la, gli, le) before a possessive adjective like my, your, his, her (mio, tuo, suo…) or you’ll end up sounding like Tarzan.
Let's see an example:
- Tarzan would say: "mio gatto mangiare crocchette" (notice that there's no "the" before mio). This is wrong ✖️
- A native Italian would say: "IL mio gatto mangia crocchette". This is correct ✔️
Exception
- Padre (father)
- Fratello/Sorella (brother/sister)
- Zio/Zia (uncle/aunt)
- Nonno/Nonna (grandmother/grandfather)
- Nipote (niece/nephew)
- Cugino/Cugina (cousin)
- Marito/Moglie (husband/wife)
- Figlio/Figlia (son/daughter)
- Suocero/Suocera (mother-in-law/father-in-law)
- Genero/Nuora (son-in-law/daughter-in-law)
- Cognato/Cognata (brother-in-law/sister-in-law)
- Mio nonno ✔️
- IL mio nonno ✖️
- Mia madre ✔️
- LA mia madre ✖️