https://www.englishforums.com/English/AGrammarQuestionUntil/nvvzv/post.htm CalifJim: There is no inversion for negation in the highlighted element of a cleft structure (It is/was not ... that ...): It was not until 2010 that the family reunited. Without a cleft structure, initial negations trigger inversion: Not until 2010 did the family reunite. _______________ You can form both of these structures for most negations. The hunters didn't return until Monday. It was not until Monday that the hunters returned. Not until Monday did the hunters return. _______________ The same applies for "only": It was only after it was too late that Jenny applied for the exemption. Only after it was too late did Jenny apply for the exemption.

Mana Ant has a great answer. For common SPOKEN American English, the correct sentence above would be usually said as follows: "It wasn't until he entered the room that I recognized him."

Guys, I've checked the key again. The true answer is "It's was not until he entered the room that I recognized him". Sorry for my mistake! Fool the book TT

Thank guys !!!