Subject/pronoun-Verb Agreement with ser
 

This is something I should have learned like my first month of Spanish, and I am embarrassed that I do not know it:

Obviously conjungation of a verb depends on the subject (Yo voy, Tú vas, etc).
With the verb ser, if you use a pronoun + ser to say that the person equals something else (predicate nominitive), does the conjugation always match the pronoun, which could be be the subject or the predicate nominitive, or does it always match the subject of the sentence, whether or not it is the pronoun?

Examples:

A. Tú eres mi felicidad. (Tú is the subject, so we choose eres.)
But, if subject and predicate nominitive are reversed:
1) Mi felicidad es tú.
--or--
2) Mi felicidad eres tú.

B. Yo soy el camarero. (Yo is the subject, so we choose soy.)
But in reverse:
1) El camarero es yo.
--or--
2) El camarero soy yo.

I would think that in both cases option 1 would be correct because the subjects are felicidad and camarero, but I recall a billboard in Santo Domingo that said, "Lo que importa eres tú," which leads me to think that ser is always conjungated to match the pronoun, regardless if it is the subject or predicate nominitive.

Thanks!

In a subject / subject complement structure, the subject and the subject complement are identical (if the subject complement is a noun). If 'I' is the subject, what 'I' am is the subject complement, in other words. By the same token, if 'home' is the subject, what 'home' is is the subject complement. The subject always 'drives' your decision for verb conjugation.

The problem is, how do you determine who the subject is? Well, that very much depends on context and, in colloquial English, the subject is often listed first. For instance, it's perfectly fine to say, "The problem is you," where "problem" is the subject. If we switch the two nouns around, the linking verb becomes 'are', to match the subject 'you'.

The same logic cannot be applied in Spanish when the subject is a subject pronoun. No matter its location, the linking verb will always agree with the subject pronoun.

Mi felicidad eres tú.
Tú eres mi felicidad.

Soy el camarero.
El camarero soy yo.

Perfect. Thanks so much!

Actually I was just thinking of times when there is not a pronoun. In those cases do you use the subject that comes first?

Example:
El problema es los estudiantes.
Los estudiantes son el problema.

El costo más grande es los trabajadores.
Los trabajadores son el costo más grande.

Correct?

It's not wrong, but I prefer

El problema es el cuerpo de estudiantes.
El problema es el alumnado/estudiantado.
El problema son los estudiantes.

El costo mayor es el de los trabajadores.

In an informal level, son is OK and es is also OK but it sounds a bit lame (cripple) because it is elevated in the language aspect but not in the exposition of ideas.

By the way, congratulations, as this question has an educated-native speaker level.