Breakdown of هذا مصدر موثوق به.
هذاhâdhâ
this (m)
هhu
his / him (object) / it (dummy pronoun after لأن)
بbi
by, with (attached)
مصدرmasdar
source
Questions & Answers about هذا مصدر موثوق به.
Why do we need به at the end of the sentence?
The word موثوق comes from the verb وثق (to trust), which requires the preposition بـ (in). When using a passive participle of a prepositional verb as an adjective, Arabic requires you to keep the preposition and attach a pronoun referring back to the noun. Literally, it translates to 'a source trusted in it'.
If I wanted to say 'these are reliable sources' (plural), how would the phrase change?
You would say هذه مصادر موثوق بها. Because مصادر (sources) is a non-human plural, it takes feminine singular agreement. The pronoun on the preposition changes to the feminine ـها, but the word موثوق surprisingly remains masculine singular!
Wait, why doesn't موثوق change to feminine singular (موثوقة) to match the non-human plural noun?
This is a special quirk of Arabic grammar. When a passive participle relies on a preposition to connect to its noun (like بـ here), the participle itself stays fixed in the masculine singular form. The agreement is handled entirely by the pronoun attached to the preposition.
Can I just say هذا مصدر موثوق and drop the به completely?
In formal Modern Standard Arabic, omitting به is incorrect because the base verb cannot take a direct object without its preposition. While you might hear it dropped in casual speech or less formal modern media, you should always include به in B2-level writing and exams.