Learn Arabic (Modern Standard): المجمع الجديد ضخم.

مُجَمَّع (mujamma') is a noun of place derived from the Form II verb جَمَّعَ (jamma'a), meaning to gather or assemble. For derived verbs (Forms II to X), the noun of place and time looks exactly like the passive participle. You form it by taking the present tense, replacing the first letter with a مُـ (mu-), and giving the second-to-last root letter a fatha (short 'a' sound). A place of gathering becomes a complex.

If المجمع follows the same pattern as the passive participle, how do we know it means a place here?

Context is everything! Because the word pattern for nouns of place in Forms II-X is identical to the passive participle (meaning something that has been gathered), you rely on the sentence to tell them apart. Here, describing the subject as new and huge points to a physical location, so it translates as a complex or center rather than 'the assembled thing.'

Why does الجديد (new) have the definite article, but ضخم (huge) does not?

This is how Arabic builds a basic nominal sentence to say 'A is B'. الجديد is an adjective describing the subject المجمع, so it must match its definiteness (both get the 'al-' prefix). ضخم, however, is the predicate (the news about the subject). Predicates in this type of sentence are typically indefinite to create the 'is' meaning. If it had the definite article, the phrase would just be an incomplete fragment meaning 'the huge new complex...'

What is the root of المجمع and what other familiar words share it?