"I think Mr. Tanaka's mother is a teacher."


July 8, 2017

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is "da" the plain form of desu in this sentence? I wasn't sure whether it was a part of the sentence structure.


Yeah - だ can be considered the plain form of です when applied to a noun, it functions similarly in expressing a state of being, and inflects in similar ways for the past and such.

You'll also see many occurrences of です though which can't be replaced by だ and which merely serve to make a sentence more polite - particularly those which follow an い-adjective. Those occurrences are also strange in that they can't be inflected:

楽しい - (It) is fun (plain)

楽しいです - (It) is fun (polite)

楽しかった - (It) was fun (plain)

楽しかったです - (It) was fun (polite)


Let me just add to that by saying that the opposite is also true, ie, there cases such as this example where だ must be used. Typically, the polite conjugation must go at the end of a sentence, and within the sentence だ must be used instead of です for state of being.

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