Learn Italian: Venezia, crocevia dell'antico commercio, era molto ricca. - Venice, the crossroads of ancient commerce, was very rich.

In Italian, when you use a noun phrase immediately after a proper noun to define or describe it (an appositive), you must drop the definite article. Since crocevia defines the proper noun Venezia, we say Venezia, crocevia... instead of Venezia, il crocevia....

Is the word crocevia singular or plural here?

It is singular here. Crocevia is a masculine compound noun (from croce and via). Like many compound nouns ending in -a, it is invariable, meaning its form doesn't change in the plural. You rely on context or articles to know if it's il crocevia (the single crossroads) or i crocevia (the multiple crossroads).

Why is the adjective antico placed before commercio instead of after it?

Placing an adjective before the noun gives it a more descriptive, poetic, or subjective tone. Writing dell'antico commercio emphasizes the historical grandeur of the trade, whereas del commercio antico would sound more like a dry, neutral categorization.

Why use the imperfect tense era instead of the passato prossimo è stata for 'was'?

The imperfect tense era is used because it describes an ongoing state or condition in the past. It sets the scene by telling us how Venice used to be over a long, undefined period, rather than referring to a completed historical event.