Learn French: Ils font étudier la modernisation de la gare. - They are having the modernization of the train station studied.

It transforms the action of a verb (moderniser) into a noun (la modernisation). Instead of using a longer clause like qu'on modernise la gare (that the station is modernized), the sentence condenses the action into a single noun phrase. This makes the statement more concise and is highly typical of administrative or journalistic French.

Why do we need de in de la gare?

When you change a verb into a noun, its original direct object needs a preposition to connect to the new noun. The original verbal idea is moderniser la gare (to modernize the station). In the nominalized version, you must link the two nouns with de, becoming la modernisation de la gare.

Why is the verb faire used here instead of just conjugating étudier?

This is the causative construction (faire followed by an infinitive), which means to have an action done by someone else. Ils étudient would mean they are personally doing the research. Ils font étudier means they are commissioning or ordering others to do the study for them.

Why is étudier an active infinitive when the English translation says 'studied'?

English uses a passive structure with a past participle for this causative meaning ('having something done'). French, however, strictly requires the active infinitive after faire. You cannot use a past participle or a passive voice here; the structure always literally translates to 'making to study' the project.