I saw this sentence in Lesson: "Este es el tercer concierto este año", and wonder why it used "tercer" instead of "tercero", while "concierto" itself is a masculine.

I am assuming that this is the same as un vs uno. We use "un" when it is used before masculine singular nouns, but we use "uno" as number / noun. Example:

¿Cuántos vestidos quieres?

Quiero un vestido.

Quiero uno.

So it will be the same for tercer and tercero as well. Example:

El tercer concierto este año.

Este es el tercero.

As for the feminine form (una and tercera), they can be used both in front of a feminine noun and as a number / noun.

~CMIIW

Hi Mega! Great observation! Very impressive how you connected the dots and landed on the correct answer by yourself 😃 Question: what does ~CMIIW mean? Is that your signature?