。
Physics is not easy to learn.
Literal
Physics [topic-は] learn [nominalizer-の] [subject-が] easy not-is.
学ぶのが簡単ではない uses のが to nominalize 'learning' and mark it as the subject of 簡単ではない (is not easy). This 'Xは verb-のが adjective' pattern is fundamental for describing the difficulty of activities.