当 vs 认为 vs 以为（近义词）

1)The three words all mean "to ascertain one's view".认为 is relatively formal,以为 is most popular,while 当 is casual and most colloquial.
2)认为 and 以为 can both be used in the contrast between subjective judgement and actual fact:
3)认为 is mostly used in positive judgement:
4)In examples③and④以为 can also be used if one wants to be more tactful.
5)以为 is often used to state a mistaken understanding and is followed by a clause indicating transition:

1)认为 can be qualified by an adverb(including negative adverbs):
2)以为 is usually qualified by a monosyllabic adverb,e.g.真以为(really think),原以为(had thought )It is restricted in being qualified by a negative adverb:either it is used in a four-character phrase(e.g.不以为然)，or it is split into 以…为…which is left over from classical Chinese:

1)当 is the most colloquial and never qualified by any adverb:
2)In examples①and②，以为 can replace 当，but is not so colloquial.
3)Note:In examples③and④，当 is qualified by a monosyllabic adverb,meaning to look on as "or "to treat as":