unimaginable, It is hard to believe/imagine …
　　
　　表示“重要”： important , critical , crucial, essential , fundamental , foremost , significant [意义重大], vital ,
　　
　　表示“不可缺少”： necessary , essential , indispensible , integral [强调组成整体中的一部分]
　　
　　* Kids are an integral part of a family.
　　
　　表示 “明显的”： obvious, apparent, clear , distinct , evident , manifest , # overt [公开的]
　　
　　表示 “复杂的”： complicated , complex , involved , sophisticated [强调因为技术精密而复杂]
　　
　　* a sophisticated telescope 精密复杂的望远镜
　　
　　表示 “有前景”： promising , potential , prospective
　　
　　功能副词：
　　
　　表示“主要地，根本上地” mainly, basically, chiefly, essentially, primarily, principally，examda.com
　　
　　表示“仅仅，只不过”： only, just , merely, solely , no more than ，exclusively
　　
　　* He learned music solely for fun. 他学音乐只是为了开心。
　　
　　* He learned English exclusively by himself. 他的英语只是自学的。
　　
　　[强调没有靠其他方式]
　　
　　表示 “非常，很”： very , greatly ,awfully, badly, considerably , enormously, extremely,
　　
　　significantly, highly, radically, excessively [过渡地],
　　
　　# disproportionately[比例过大地],
　　
　　表示“大约” : about, approximately , around , or so ,
　　
　　some: Some 4000 people went to the show
　　
　　表示“实际上，实质上”：
　　
　　actually , as a matter of fact , in fact , in effect , practically, really, virtually
　　
　　其他
　　
　　表示“关于”：about, in regard to , respecting , on , with respect to ,
　　
　　in terms of（在…方面）：China is great nation in terms of its history.
　　
　　表示“同时”： at the same time , in the meantime , meanwhile ,
　　
　　#simultaneously, synchronously
　　
　　表示“根据”：according to , based on ,
　　
　　in the light of : *In the light of the evidence, we are convinced that he is right.
　　
　　表示“暗示，线索”： clue , clew, hint , implication