Learn French: Nous voulons cette archive, mais ils en ont besoin.

In French, the expression for 'to need' is avoir besoin de. Because it requires the preposition de, you must use the pronoun en to replace de plus the object (cette archive). You use direct object pronouns like la only when the verb takes its object directly without a preposition.

Why is en placed before ont instead of before besoin?

Even though avoir besoin translates as a single verb 'to need' in English, grammatically it is the conjugated verb avoir (to have) followed by the noun besoin (need). In French, object pronouns like en must go directly before the conjugated verb, which is ont in this case.

If the sentence started with 'We want it', would that also use en?

No, it would be Nous la voulons. The verb vouloir (to want) takes a direct object, meaning it doesn't use the preposition de. You only use en to replace an object when the verb or expression specifically requires de.

Why is it cette archive and not ce archive?