I have some problems with understanding what is the difference between these
two. According to 初級日本語, 間 refers to certain time frame whereas 間に refers to
certain time frame within that time frame. Is it correct or am I missing
something?
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2013-08-13T11:34:54.183
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2021-02-03T01:19:43.353
2021-02-03T01:19:43.353
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question
grammar word-choice particle-に time
Difference between 間 and 間に
I've summarized the following from a _Dictionary of Basic Grammar_ which seems
a little different from what you state. (Note that I'm not a native speaker.)

The time span in the clause before 間 and the main clause is the same, whereas
the time span in the clause before 間に is wider than the time span of the main
clause.

For example:

> 山田さんが巴里｛パリ｝に留学している間にお母さんが病気になった。

The period in which she became sick is a portion of the time when Yamada was
studying in Paris. If 間に is replaced by 間 then the sentence becomes
ungrammatical according to _A Dictionary of Basic Grammar_.
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2013-08-15T00:48:13.717
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2016-11-04T18:58:23.960
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answer
It is very simple:

A 間 B

means;

All the time that A took place B was also happening. (I read my book while it
was raining)

A 間に B

means;

While A took place B happend. The nuance to appreciate is that B is an event
that occurred (started & finished) at some point during the period A took
place. It did not go on all the time. (While it was raining the post was
delivered.)

The nuance is very similar to the difference between まで and までに

A まで B

means;

Until A happens, B will continue, constantly (I was watching television until
father came home)

A までに B

means;

By the time A happens B will have occurred (started and finished, during the
period defined by A まで; it could happend at any point but did not happen
constantly. (I will finish my work by 5 o'clock. ie at some unspecified point
during the period to 5'oclock)
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2013-08-15T14:02:14.850
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2013-08-17T01:05:01.717
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answer