Answer:
If he keeps on forgetting it, the teacher will punish him.
Step-by-step explanation:
Conditional sentences are characterized as the sentences that display a condition or hypothetical situation followed by its associated outcome. Such sentences consist of a dependent(if) clause which expresses the condition and an independent clause to show the consequence.
As per the rules of the conditional sentences, if the consequential clause is written in future simple, the conditional clause would be written in present simple.
'If + present simple(V1 + s/es); main clause + future simple(will/shall + V1).'
Thus, the correct verb form to fill the given sentence would be 'keeps'(present simple) followed by future simple('will punish').