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。

youdao

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。

youdao

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。

youdao

But protecting plants and trees in the garden may not be too difficult if you follow a few suggestions.

VOA: special.2010.01.12

Journalists get the training and practice to be able to do this speedily every day, but it's perhaps not too difficult to learn, and allowing other people to edit and improve your work can help overcome sloppy writing or typos.

BBC: NEWS | UK | Magazine | Citizen journalist: My week

It's not too difficult, but is still considerably more complicated than walking into the library and pulling a book off the shelf.

CNN: E-books spur reading among Americans, survey shows

Difficult qualifying as we were not too quick yesterday but we knew we had the pace on the long runs.

NPR: Alonso Thrills Home Fans With Spanish GP Win