回答

1. (It takes a green thumb) to grow cabbages as big as that.
※形式主語構文:It 形式主語/to grow … 真主語  green thumb 植物栽培の才能
2. The medicine bottle was designed (so that it couldn't be opened) by children.
※so that+S+can ~ Sが~できるように
3. The decline in general enrolment (has made it difficult for) private universities to stay open.
※… has made it(形式目的語) for private universities(不定詞の意味上の主語) [to stay open](真目的語).

参考にしてください。

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