Learn Romanian: Noi o invităm pe prietena lui la restaurant. - We are inviting his friend to the restaurant.

The o is a short pronoun meaning "her". In Romanian, when the direct object is a specific person (like "his friend"), we use this short pronoun right before the verb to point to them, even though we also name the person later in the sentence. It is basically like saying "We her invite his friend".

Why do we need the word pe before prietena?

Pe is a special marker used when the receiver of an action is a specific, known person. It doesn't translate to an English word here, but combined with the o before the verb, it is the standard, required way in Romanian to say you are doing an action to a specific human.

Why does the sentence use prietena instead of just prietenă?

Because we are talking about a specific friend ("his friend"), Romanian requires the definite article. The -a at the end of prietena is the equivalent of "the". When you use pe to point out a specific person followed by a possessive word like lui, the noun must take this definite article.

Possessive words usually match the noun's gender. Why doesn't lui change to match the feminine noun prietena?

Lui (meaning "his") is a great exception to remember! Unlike the words for "my" (meu/mea) or "your" (tău/ta), the word lui never changes its shape. It stays exactly the same whether the person or thing being owned is masculine, feminine, singular, or plural.