Why is there "Il" in "Il mio tesoro intanto"? Isn't "Il" just definite a article and thus not needed?
Italian, unlike French, English or German, does not treat the possessive adjectives as determiners. They are ordinary adjectives and so are normally prefaced by the article when appropriate.
In particolar mio in Il mio tesoro intanto ("Meanwhile, my treasure") works exactly like dolce in Il dolce tesoro intanto ("Meanwhile, the sweet treasure").
Note that this allows one to use the indefinite article un,uno as well, which in English is impossible and requires the usage of periphrases to be expressed. For example
Un mio amico (A friend of mine)
Italian wants the article with possessive adjectives and pronouns.
The article is omitted when the possessive refers to relations: mia madre, mio padre, mio fratello, mia sorella, mia zia, mio cugino and so on. With madre and padre the noun is always omitted, with other relations the name can be added: mio fratello Asdrubale. It is also omitted in vocatives: Tesoro mio, ti amo; it can also be mio tesoro, ti amo.
It is also omitted in special phrases: casa mia (that corresponds to the English my home), but la mia casa (which is my house).
Don Ottavio is singing
Il mio tesoro intanto
andate a consolar.
E del bel ciglio il pianto
cercate di asciugar.
The article is used with mio and bel1 (fused with the preposition di, as usual).
Among Romance languages, Italian is not unique for this: Catalan and Romanian use the article (postponed in the latter); French and Spanish don't. Related: https://linguistics.stackexchange.com/questions/26261/why-does-italian-use-definite-articles-before-possessive-adjectives-except-when
1 The adjectives bello and quello have peculiar forms:
- bel and quel are used when before a name that takes the article il
- bello and quello are used when before a name that takes the article lo
- bei and quei are used when before a name that takes the article i
- begli and quegli are used when before a name that takes the article gli
The forms are regular for the feminine (bella, quella, belle, quelle) and when the masculine adjective is postponed or bello is used as aggettivo sostantivato.