In a way you will never forget. It is the adverb from 'unforgettable', so it carries the same feeling: something hits you so hard — usually because it was beautiful, or wonderful, or shocking — that the memory stays with you for years without you trying. Most of the time it is warm and positive: an unforgettably good meal, an unforgettably kind teacher. But the word itself is not only for good things; people also say a night went unforgettably wrong, and then it means the badness is what sticks. Notice that it is a bit of a strange adverb in practice. English speakers say 'unforgettable' far more often than 'unforgettably', because the adjective can sit right in front of the noun ('an unforgettable trip'), while the adverb needs an adjective or a verb to attach to ('unforgettably beautiful', 'she sang unforgettably'). So the most natural use is before another adjective, as a kind of strong 'very'. It feels a little literary or advertising-ish — you meet it in travel writing, film reviews, and hotel brochures more than in daily chat. In everyday talk a Taiwanese student's friends would more likely say 'I'll never forget it'.

令人難忘地、難以忘懷地。要注意這是副詞，中文常常會直接說成「令人難忘的」，那是形容詞 unforgettable；副詞多半用在另一個形容詞前面，例如 unforgettably beautiful ＝ 美得讓人忘不了。