comprehend|comprehended|comprehending

我花了一些时间才完全理解这个概念。

It took me some time to fully ___comprehend this concept.

他终于完全理解了那道数学题的解法。

He finally ___comprehended the solution to the math problem.

小孩子很难理解复杂的成人问题。

Young children have difficulty ___comprehending complex adult issues.