Learn French: Il faut entretenir la berline pour garder sa fiabilité. - You have to maintain the sedan to keep its reliability.

In French, entretenir is used for the physical upkeep or maintenance of a machine, building, or relationship. Maintenir means to hold something in place, enforce, or keep something at a certain level, like maintenir l'ordre (to maintain order) or maintenir la pression (to maintain pressure).

The English translation says 'You have to', but the French uses Il faut. Could I say Tu dois here?

You could, but il faut followed by an infinitive is the most natural way to express a general obligation or a rule in French. It translates to 'it is necessary to' or a general 'you have to' or 'one must', making it perfect for general advice about car maintenance where 'you' doesn't refer to one specific person.

Why is it sa fiabilité for 'its reliability'?

In French, possessive adjectives agree with the noun being possessed, not the owner. Fiabilité is a feminine noun, so it requires the feminine possessive sa. Unlike English, French doesn't have a separate word for 'its' versus 'his' or 'her'—the words son, sa, and ses cover all three depending on the gender and number of the object being owned.

What exactly is a berline? Can I just use voiture?

Voiture is the general word for 'car', but a berline specifically refers to a 'sedan' (or 'saloon' in UK English). It's a useful vocabulary word to know when distinguishing between car body types, much like knowing how to say SUV or convertible.