Расщеплённые предложения
What I need is… / It was then that…
Конструкция вытаскивает нужное слово в центр внимания:
- What I need is a good night\'s sleep.
- What happened was that the file got deleted.
- It was then that I realised my mistake.
- All I did was ask a question. — после all + do идёт голый инфинитив.
- The reason why he left was the salary.
В устной речи то же самое делает интонация — на письме нужна конструкция.