Learn Arabic (Modern Standard): السوق مزدحم في العصر. - The market is crowded in the late afternoon.

In Modern Standard Arabic, سوق is actually one of the few nouns that can be treated as either masculine or feminine! Here, the writer chose to treat it as masculine, which is why the adjective مزدحم doesn't have the feminine ة (taa marbuuTa) at the end. Either السوق مزدحم or السوق مزدحمة would be grammatically correct.

What is the root of مزدحم (crowded), and why is there a د (daal) in it?

The root is ز ح م (z-H-m), relating to crowds or pushing. مزدحم is a Form VIII active participle (a word acting as an adjective to describe who or what is doing the action). Normally, Form VIII adds a ت (taa) after the first root letter. However, there is a special spelling rule in Arabic: when the first root letter is ز (zayn), that added ت turns into a د (daal) to make it easier to pronounce. So instead of 'muztahim', it becomes مزدحم (muzdaHim).

How exactly does العصر translate to 'late afternoon'? Isn't afternoon usually بعد الظهر?

While بعد الظهر literally means 'after noon' and covers the early afternoon, العصر specifically refers to the late afternoon. This time period is culturally and linguistically significant because it corresponds to the time of the Asr Islamic prayer, typically starting when the sun is roughly halfway down from its peak and ending at sunset.

If I were reading this with full grammatical vowels (tashkeel), what would the word endings sound like?