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Understandable 有被动意味。起逻辑主语往往是他人（他事）。
字典定义：
Understandable: capable of being understood; comprehensible.
例句：
1。 Notice that only the student is the only one who vented that level of frustration at you---and this is understandable.
3。The argument made in this article is logical and understandable.
3。 Their scepticism is understandable, given the government's general record of wastefulness.
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understanding“ 有主动意味。
字典定义：
understanding:
1. adjective, characterized by understanding; prompted by, based on, or demonstrating comprehension.
2. adjective, sympathetic, tolerant, or wise towards people
3. adjective, possessing judgment and intelligence
例句：
"she nodded understandingly"
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你的原句：
"The man smiled back, pointing his finger to the TV and said, “He (Obama) is very smart. I like him.” I never knew how to follow a conversion and keep it alive. So, I smiled understandably and remained silent. After several minutes of soliloquising, the man started asking me questions, in an apparent effort to look for new topics."
显然这儿用understandingly好。
