Student or Learner
How come in the following sentence When a person learns English as a Second Language, they are speaking there's the continuous form (speak+ing) in the second part? And if someone decided to use it, how come someone didn't say "when a person is learning English..." ?
In any case, there is no compulsion to use the same form in all the clauses in a sentence. It would be just as valid to ask "If you use the simple form in the first clause, how come you also have to use it in the second?" - to which the answer would be, "You don't".