Learn Russian: Они хотели бы заказать десерт, потому что он вкусный. - They would like to order dessert, because it is delicious.

To say "would like" politely in Russian, we use the conditional mood. You form this by combining the past tense of the verb with the particle бы. Since the subject is Они (they), we use the plural past tense form хотели.

Why is заказать (perfective) used here instead of заказывать (imperfective)?

When expressing a desire to complete a specific, one-time action with a concrete result—like ordering a dessert at a restaurant—you use the perfective verb.

Is десерт in the Nominative or Accusative case here?

It is in the Accusative case because it is the direct object being ordered. Because десерт is a masculine inanimate noun (an object, not a living being), its Accusative form looks exactly like its Nominative dictionary form.

Why does the sentence use он ("he") if we are talking about dessert ("it")?

In Russian, the pronoun must match the gender of the noun it replaces. Десерт is a masculine noun, so we use он to mean "it" here. If we were talking about a feminine noun like пицца (pizza), we would use она and say она вкусная.