Use material that is challenging, but not too difficult.
使用的学习材料要具有挑战性，但不要太难了。
Ruby very rare precious natural, but not too difficult, so artificial industrial rubies are artificial.
天然红宝石非常少见珍贵，但是人造并非太难，所以工业用红宝石都是人造的。
Kids need some fat, too, but that's not usually difficult to get.
孩子们同样需要一些脂肪，但这通常不会很难得到。
That's not too difficult to do, but the implementation code becomes fairly complex.
这并不太难做到，但是实现代码将变得相当复杂。
Not too difficult an example, but one that clearly demonstrates the point!
不是太困难的例子，而是一个清楚地表明了这一点！
But I would not like to learn, because it's too difficult.
但是我并不想学，因为英语太难了。
But I would not like to learn, because it's too difficult.
但是我并不想学，因为英语太难了。
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