@TTSM Not exactly. Suddenly means "without warning", like it's a surprise that something happened. Instant is closer to the word moment in meaning. If you said "Suddenly, she was gone", it means she disappeared and it was unexpected. Saying "She was gone in an instant" means she disappeared but it may or may not have been without warning. "Suddenly she seemed happy" doesn't have the same meaning as "For an instant she seemed happy." The first one means "Without any warning, she seemed happy." but the second means "For a single moment, she seemed happy". I hope that makes sense. Also I'm sorry that this is so long!