"This concept is not new."
Translation:Este concepto no es nuevo.
I still don't understand the este/esta/esto, is there actually a rule for that?
Never use "esto" right before a noun. And "esta" (feminine) and "este" (masculine) must agree in gender with the noun they come before. When the noun does not come right after or there is no noun/pronoun is where it can get confusing. Questions like "What is this?" can be answered with "esto". Esto es un pato. Even though we know "esto" refers to a masucline noun "pato". Things with no gender use "esto". This explains your motives. Esto explica sus motivos.