When should the subjunctive be used after “mientras”?

Question

When should mientras or mientras que be followed by the subjunctive and when by the indicative? In both cases, how is mientras usually translated?

Answer 1

Subtle...

Mientras llueve, escucho música. (llueve: indicative)

While it rains, I listen to music

Mientras llueva, me quedaré en casa. (llueva: subjunctive )

As long as it rains, I'll stay at home.

"Mientras + indicative" means mere simultaneity (at the same time something happens, another thing takes place). "Mientras + subjunctive" suggest a condition (as long as something is happening ...)