题目是这样的：All ______ he did was to hear from his friend as soon as he arrived.这里的空白处需要填写正确的动词形式。解答这类题目时，我们需要考虑句子的时间关系和语境。在这个例子中，“he did”表示过去的动作，因此我们需要使用过去时态来填写空白处。正确的答案是：All that he did was to hear from his friend as soon as he arrived。这样，句子表达的意思就是：他所做的一切就是一到就给朋友回信。类似的练习能够帮助我们更好地掌握英语语法，提高阅读和写作能力。通过反复练习这些基础题目，可以逐步提升自己的英语水平。