Learn Arabic (Modern Standard): ما إن انهمر المطر حتى تجمهر الناس داخل المطعم. - No sooner had the rain poured down than the people gathered inside the restaurant.

The structure ما إن ... حتى (no sooner ... than) naturally sets up a strict sequence of events. Because the paired particles themselves convey the meaning of one thing happening immediately before another, Arabic just uses the simple past tense for both verbs. You do not need a compound tense like كان قد to express the English past perfect 'had poured'.

The verb تجمهر (gathered) looks like a Form V verb, but the root seems to have four letters. How does that work?

You have a good eye! It behaves exactly like a Form V verb, but it is built from a four-letter (quadriliteral) root: ج م ه ر. It is actually derived from the noun جمهور (crowd), so the verb literally means to form a crowd or to mass together.

الناس (the people) is a plural noun. Why does the verb تجمهر remain in the singular masculine form?

This is a classic rule of Arabic sentence structure that is always good to remember. When the verb comes before its explicit subject, the verb stays in the singular form, even if the subject is plural. Since الناس is a collective noun treated as grammatically masculine, we use the singular masculine تجمهر.

What is the grammatical case of داخل (inside) and the word following it, المطعم (the restaurant)?