The verbs to doubt (dudar) and to deny (negar) take the subjunctive when they are affirmative, but not when they are negative.
I doubt he'll come.
Dudo que venga.
I don't doubt that he'll come
No dudo que viene
The Spanish subjunctive is required after a number of common conjugations and relative pronouns, whereas its usage is optional after others.
Conjunctions which require the subjunctive:
on the condition that
a condición de que
so that
a fin de que
unless
a menos que
before
antes (de) que
provided that
con tal (de) que
Conjunctions of time. take the indicative when the action in the subordinate clause is either habitual or in the past. The subjunctive is used when the main clause is a command or in the (potential) future:
as soon as
así que
when
cuando
after
despues de que
as soon as
en cuanto
until
hasta que
as soon as
luego que
as soon as
tan pronto como
I worked until my kids called.
Trabajé hasta que mis niños me llamarón.
I (usually) work until my kids call.
Trabajo hasta que mis niños me llaman.