Learn Arabic (Modern Standard): هذا عمل كامل.

In this sentence, كامل (complete) is an adjective describing the noun عمل (work), not an adverb. It matches the noun perfectly. If you wanted to say 'completely' as an adverb of manner (like 'I read it completely'), you would use the accusative form كاملاً that you are learning in this lesson.

Where is the word for 'is' in this sentence?

Arabic does not use a word for 'is' in simple present-tense sentences like this. By placing the pointer هذا (this) right before the indefinite phrase عمل كامل (a complete work), the language automatically creates the equational meaning 'This is a complete work.'

Why is the word order عمل كامل instead of the other way around?

In Arabic, adjectives almost always come after the noun they describe. So instead of saying 'complete work', you say the noun عمل (work) first, followed by its adjective كامل (complete).

If I were to read this sentence with full formal pronunciation, what would the case endings be?

Fully pronounced, it is 'haadhaa amalun kaamilun'. Because عمل and كامل are indefinite and form the core information (predicate) of the sentence, they both take the default nominative 'un' sound (tanween damma) at the end.