tīng / wán / lǎo / shī / de / jiě / shì / tā / huò / rán / kāi / lǎng / zhōng / yú / míng / bái / le / wèn / tí / de / guān / jiàn

听完老师的解释，他豁然开朗，终于明白了问题的关键。

After hearing the teacher's explanation, everything suddenly became clear to him, and he finally understood the key point.