Question regarding the use of "is" versus "are"

[lillyb]

I am quite confused about when I should use "is" instead of "are" in a sentence. I know that "is" should be used when nouns joined by "and" are thought of as a unit, but I have trouble identifying such cases.

e.g. 1 The sum and substance of the matter is clear.

The previous example is pretty straightforward, in that "the sum and substance" obviously go together, so they can be referred to as a single aspect.

e.g. 2 Ice cream and cake seem/seems to be essential for any child's birthday party.

"Ice cream and cake seem" sounds natural to me, but I am not sure. They can either be considered to be "a dessert", in which case the singular should be used, or they can be regarded as separate dishes, which requires the plural.

If someone could explain this, I would be really grateful.

[Bennevis]

Ice cream and cake seem/seems to be essential for any child's birthday party.

These are two separate items; therefore, "seem" is correct. It's not one dish or items necessarily served together.

[Bennevis]

A possible workaround for this issue is paraphrasing that part of the sentence:
Serving ice-cream and cake seems essential ...

[lillyb]

Thank you very much!