Learn Arabic (Modern Standard): يبيع الصائغ الألماس.

In Arabic, when you refer to a substance, category, or abstract concept in general (like diamonds, gold, or love), you must use the definite article ال. If you left it off, it would imply 'some diamonds' rather than diamonds as a general trade good.

What type of verb is يبيع (he sells), and why does it have a long 'ee' sound in the middle?

The verb is from the root ب ي ع. It is a 'hollow' verb, which means its middle root letter is weak. In the present tense, this weak middle letter naturally appears as the long vowel ي (ī), making the word يبيع (yabīʿu). The past tense is باع (bāʿa).

The word for jeweler, الصائغ, follows a very common noun pattern. How is it constructed?

It follows the active participle pattern, which is used to describe the person or thing doing an action. It comes from the root ص و غ (to mold or craft). By putting it into this pattern, it literally translates to 'the crafter' or 'the one who crafts'.

If I wanted to pronounce the full case endings (the short vowels at the ends of the words) for this sentence, what would they be?

The subject الصائغ is in the nominative case, so it takes a 'u' sound (ḍamma) at the end: aṣ-ṣāʾighu. The object being sold, الألماس, is in the accusative case, so it takes an 'a' sound (fatḥa): al-almāsa.