Learn Russian: Обособление требует внимательности.

Outside of grammar, it means 'isolation' or 'setting apart'. But in Russian punctuation rules, обособление is the specific term for 'setting off with commas'. In this lesson, it is reminding you that correctly placing commas around verbal adverbs (деепричастия) takes careful thought.

Why is внимательности in the genitive case rather than the accusative?

The verb требовать (to require or demand) usually takes its object in the genitive case instead of the accusative, especially when the object is an abstract concept like внимательность (attentiveness). You might see the accusative used for demanding a specific, concrete physical object, but for abstract requirements, always use the genitive.

Could I just say требует внимания instead?

Внимание (attention) and внимательность (attentiveness or carefulness) are very close in meaning, but they have a slight nuance. Внимание is the general act of focusing, while внимательность is the personal quality of being observant and detail-oriented. Because applying punctuation rules requires you to be detail-oriented, внимательности is a more precise fit here.

Why does the genitive form of внимательность end in -и?