2014年4月19日 · I guess this question has been asked before, but please take a look the following sentence and tell me if there is a difference between them. When the transaction had been completed, A was still a

2012年7月1日 · Possible Duplicate: “Did it close” vs “Has it closed”? As a English non-native speaker it is difficult for me to understand when I must use present perfect or past simple

In your specific case, neither 'was' nor 'were' is best; you should say "if it is running". "If it were running" is subjunctive case, used to describe hypothetical situations: "If it were running, I would stop it first, but it's already stopped."