〜（は）〜のひとつだ

About

The pattern 「XはYの一ひとつだ」 uses the は particle to establish a topic (X) and says that it is one of many within a given category (Y). If the category is comprised of people, 一人ひとり will be used instead of 一ひとつ. Other counters are also used when appropriate, such as 〜冊さつ for books, or 〜枚まい for flat objects.

は may substitute for が in situations where the speaker wants to place emphasis on the subject preceding が. は may substitute for も when the speaker wants to add something to a list of things that exist within a category.

Summary

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  〜は(category)の一つひとつ(だ・です) is used to express that something is one part of a larger category of things

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  The counter used can vary based upon the category