Is it true that if someone misses 'ishā' prayer then he/she cannot offer tahajjud prayer the same night?
We believe that Isha prayer is obligatory and tahajjud is supererogatory. None would offer tahajjud before offering the isha prayer. He would like to first offer the isha prayer and discharge his primary duty before embarking on an optional good deed of the same sort. So the question whether one can do so or not is not relevant. I would say none would do that even if they are slightly familiar to what they are doing.