The list of 'conjunctions' you gave ALWAYS call for the subjunctive mood in the clauses they introduce and the fact that used them imply that there IS a change in subject. This is the rule you should tell your students.
Without the conjunction 'que' there is no clause and there is no verb to conjugate.
This form implies that there IS NO change in subject.
It is that simple.
Ellos trabajan para no aburrirse. (no change in subject - the people working are the same ones who don't want to get bored)
Ellos trabajan para que no se aburran. (change in subject - the people in the main clause are working so that someone else doesn't get bored)
A native speaker would not use the second sentence if the people in the main clause are the same people in the second clause.
So, you can tell your students to use the construction found in the first sentence if the subject isn't changing and the construction found in the second sentence when the subject is changing. My
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