I´ve noticed there´s quite a bit of confusion with the imperfect subjunctive so I thought I would write what I know about it.
It is imperfect because the action wasn`t completed at the time of describing it:
"Si entrara estaría adentro". As you can see i didn´t go in, otherwise it would have been simple past ("Entré") or past perfect ("habìa entrado") or some other past tense or present tense (in case i was performing the action at that time - "estoy entrando").
The tricky part is that the Imperfect subjunctive accepts two forms. -ar, -er, -ir and -se. So "entrara" (verb "entrar"), can be replaced by "entrase". This is hard to understand as there is no rule explaining it.
I have read that the second form (-se) is not as usual as the first one. In my experience (native speaker) this is not the case in Spain and most of Latin America (at least not among well educated people), where both forms are used interchangeably. So, even though it´s not necessary to use it, is good to keep in mind that you might hear it quite often!