Learn Arabic (Modern Standard): هذه الوثيقة تحتاج إلى توثيق من المصلحة. - This document needs authentication from the department.

They both come from the root و ث ق which relates to trust, confidence, or reliability. الوثيقة is the noun for the document itself, while توثيق is the Form II verbal noun (masdar) meaning the act of making something reliable, which translates here as authentication or notarization.

Why is the preposition إلى used after تحتاج? Doesn't the verb just mean needs?

In Arabic, the verb احتاج (to need) is strictly intransitive, meaning it cannot attach directly to an object. You must always use the preposition إلى right after it to introduce what is needed, functioning similarly to the English phrase has a need for.

Could we change the word order to تحتاج هذه الوثيقة instead of starting with هذه?

Yes. Starting with the verb creates a verbal sentence (جملة فعلية), which is the standard, neutral word order in Arabic. By starting with the noun phrase هذه الوثيقة to make a nominal sentence (جملة اسمية), the speaker places slight emphasis on this specific document as the main topic.

I learned that المصلحة means interest or benefit. Why is it translated as department here?