A: I know Ms. Lee was always patient with you in math.
She helped you to work out the answers yourself no matter how difficult they were.
B: Yes, and Mr. Brown guided me to do a lot better in science. He always took the time
to explain things to me clearly whenever I couldnt understand anything.
问题一，time前的定冠词，我认为用零冠词更自然，表示泛指意义。除非说话人认为听话者知道，明白这里花费的time是哪段或哪些时间。
问题二更重要，最后的一句中的anything，我觉得应该改用something更合适，因为根据这里语境的表意，应该是是说话者B无论何时遇到不懂的某问题不懂或不理解时，Mr. Brown 总会向说话者B解释清楚，所以应该是...whenever I couldnt understand something,而不是全面的否定...couldnt understand anything(...could understand nothing).
以上是我个人的看法，盼望，刘等专家给予指导，十分感谢您。