(a): “When Bobby comes” is
a time clause.
When + subject + verb = a time clause.
Will or be going to is NOT used in a time clause. The meaning
of the clause is future, but the simple present tense is used.
A time clause begins with such words as when, before, after,
as soon as, until, and includes a subject and a verb. The time
can come either at the beginning of the sentence or in the second
part of the sentence:
When he comes, we’ll see him.
We’ll see him when he comes.