

あの日のことは決して忘れません。
彼は決してうそをつかない人です。
夜の山には決して一人で行かないでください。
"けっして(決して)" is in the lesson "Time 3" (Level 40), along with 13 other useful words. LinDuo has 180 topic lessons like this one, 2,379 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.