lǎo / shī / xiān / bǔ / chōng / le / bì / yào / de / yù / bèi / zhī / shi / zài / kāi / shǐ / jiǎng / xīn / nèi / róng

老师先补充了必要的预备知识，再开始讲新内容。

The teacher first added the necessary background knowledge, then began explaining the new content.