Learn French: Il y a trois projets possibles, dont l'un améliore l'accessibilité. - There are three possible projects, one of which improves accessibility.

In French, dont is frequently used to introduce a subset or a part of a larger group that was just mentioned. Here, the larger group is the trois projets, and dont l'un isolates a single project from that group, translating perfectly to "of which one" or "including one that".

Why do we use l'un instead of just the number un?

When "one" acts as a pronoun referring back to a specific group, French strongly prefers l'un (or l'une for feminine nouns) over just un. The l' doesn't literally translate to "the"; it is simply used to show that un is standing in for a noun (in this case, un projet).

Is it strictly necessary to use l'un, or could I just say dont un?

Saying dont un améliore l'accessibilité is grammatically correct and you might hear it in casual speech. However, adding the l' makes the sentence sound much more elegant and natural, which is why it is highly preferred in written and standard spoken French.

Why is the verb améliore in the singular when the first clause talks about three projects?

The subject of the verb améliore is l'un, not the plural projets. Because l'un isolates a single project to be the subject of the second half of the sentence, the verb takes the third-person singular form.