El problema es (SER)

La diferencia es...
In Spanish, the verb ser (to be) works slightly differently to how we would use it in English.
For example: El problema son dos cosas.

In this sentence, the verb “to be” is in the 3rd person plural to agree with the 2 cosas, even though the noun is singular. This is because in Spanish ser agrees with what follows, whereas in English we would say “The difference is 2 things” or “2 things are the difference”.
Further examples:
“el chico mas guapo del mundo eres tú”,
“el importante serán la reacción y los comentarios del público”,
“lo más importante son los ascensores”,http://internacional.elpais.com/internacional/2012/04/18/actualidad/1334761565_054588.html
“el peligro de la excursión eran los tigres”.

This is equally true in the 1st person. For example on the phone if you want to say “Hi, it’s Amy” you would say “Hola, soy Amy” in Spanish!