"I work in this library."
Translation:Yo trabajo en esta biblioteca.
It's esta because biblioteca is feminine. There are 3 forms for singular this:
este - this (masculine)
esta - this (feminine)
esto - this (gender neutral)
Esta is feminine for this. Este is masculine for this. Esto is nuetral for this. I believe in this sentence the -this- is connected to biblioteca, which is feminine so it takes the esta form. Thats my guess, im sure someone else can word it better or have a better answer though.
Biblioteca is feminine, so you need to use the feminine adjective esta for it.
I agree with Yesminjdidi .. why is the is it wrong to leave off the pronoun here
Yesminjdidi is not suggesting to leave off the pronoun, but rather to mix the masculine esto with the feminine biblioteca. When you are translating and the original sentence has a pronoun, you don't leave it lost in translation (unless it is music or poetry where flow of the words is as important as accuracy of the words).