レッスンを予約するのが難しい を英語に訳すと？

「決まった」というのは「あらかじめ決められている」という意味ではなくて、「前と同じ」という意味ですよね。以下、参考にしてください。

It's hard to make an appointment with the teacher I have had before.
It's hard to make an appointment with the teacher I like.
It's hard to make an appointment with the teacher I’d like to learn from.
It's hard to make an appointment with the teacher I’d like to work with.

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