填when，句意：一天他正在打篮球，这时他突然跌倒，摔断了腿。

when是并列连词，用来构成并列句，意为“就在这时；突然”，相当于but (and, just) then。when后常接副词suddenly以强调其后并列分句动作的突然性，有使人意想不到的含义。注意：when所连接的并列分句不能位于句首。例如：

She had just lain down, when (suddenly) the baby woke up. 她刚刚躺下来，婴儿突然醒了。

We were watching TV, when the power went out. 我们正在看电视，突然停电了。