Why can't i say (the first thing teachers should do is preparing lesson plans) ?

Question

Why can't i say (the first thing teachers should do is preparing lesson plans) ?

Answer 1

Could you add more details to your question to explain what you are asking?

Answer 2

No, the first thing teachers should do is prepare routines and procedures (such as "Are the students allowed to leave class to go to the bathroom?"  "How do students hand in papers?"  "What if a student needs a pencil?"  "Where would the students sit?").

Unless you mean that your grammar in the sentence is incorrect.. Then it should be "The first thing teachers should do is prepare lesson plans."