What happens if I forget to take my inh pill?

How often? Skip it once, probably not much. Skip it more often and the tuberculosis germ can become resistant to the inh, makes it worse for you (won't kill it off), and for the rest of us (development of multiple drug resistant tuberculosis). It is a real public health problem. You must take your meds.
Once/nothing. Missing one pill will not matter. Start taking it again as soon as you realize you missed a dose. Try not to miss another dose. If you repeatedly miss doses you risk treatment failure and the development of inh resistance in the tb.