Learn Russian: Этот феномен представляет собой сущность проблемы. - This phenomenon represents the essence of the problem.

Why is сущность in the accusative case here instead of the instrumental, since verbs meaning 'to be' or 'to represent' usually take the instrumental?

The phrase представлять собой literally means 'to present by/with oneself'. The verb представлять is a standard transitive verb, meaning its direct object (сущность) must take the accusative case. It is actually the reflexive pronoun собой that is sitting in the instrumental case to complete the 'represents in itself' meaning.

If сущность is in the accusative case, why doesn't its ending change to -у?

Сущность is a feminine noun ending in a soft sign (-ь), which means it belongs to the 3rd declension (like вещь or ночь). Because it is an inanimate noun, its accusative form is exactly the same as its nominative form.

Could I just use является instead of представляет собой in this sentence?

Yes, but the grammar would have to change. You would say является сущностью, putting the object in the instrumental case. Представлять собой is slightly more formal and often used in academic or analytical texts to mean 'constitutes' or 'amounts to', emphasizing the structural nature of the subject.

Can I change the word order of the set phrase and say Этот феномен собой представляет сущность?