应当 vs 应该 vs 应（近义词）

1)应，应当 and 应该 all mean"to have the moral duty to do something".应该 and 应当are colloquial and can also be used in literary language,but 应 is used in literary language only.All the three words are.negated by 不.e.g.:
2)In examples①~⑤the three words interchangeable.but with different colour.
3)In examples⑥and⑦应当(/应该)is followed by a clause,and 应 cannot be used instead.Such clauses are limited to short ones.If they are long,由 is sually added,e.g.会议应当(/应该)由校长亲自主持(The meeting should be chaired by our rincipal),and 应 can also be used in such a case.

1)应当 or 应该can be an answer by itself,but 应 cannot:

1)应当 or 应该 also shows that the situation is reasonable to be so.应 cannot be so used.e.g.:

1)In some four character phrases,only 应 is used and not 应当 or 应该，e.g应有尽有(have everything that one expects to find),理应如此(ought to be like this),etc.