"He was playing when he fell" or "he fell when he was playing"?

Which one is correct?

He fell down when he was playing in the field.
He was playing in the field when he fell down.

Why?

Solution 1:

Both are correct; you would choose according to context.

The first suggests that the fall is the topic (i.e. you are explaining the circumstances of a fall, for example to a doctor) while the second suggests his play is the topic (you are describing his play in the field, and mention the fall as the next event in a sequence.)

The ambiguity of the sentence structure arises because "when" has slightly different meanings in the two contexts. In the first sentence, you use "when" to introduce the answer to a potential question:

When did he fall down?

He fell down when he was playing in the field.

In the second sentence, the word "when" is actually used in place of the now-uncommon "whereupon," meaning "at which time":

He was playing in the field, whereupon he fell down.

In speech however, the second sentence could convey the same meaning as the first, provided emphasis is place on the first clause, especially the gerund and noun playing and field. In this case the sentence-reversal is used to emphasise the circumstance of the fall by bringing it to the beginning of the sentence.

Solution 2:

Grammatically, both are equally correct, but stylistically, the second construction can be used for greater effect. For example:

• I was walking down the street with my mom when she got hit by a truck.
• He was playing piano when he dropped dead.

Compare that with:

• My mom got hit by a truck when we were walking down the street.
• He dropped dead when he was playing piano.