Walking down the street

[Bassim]

I am wondering if my sentences are grammatically correct.

Walking down the street, I saw a young, barefoot boy sitting on the pavement. He sang a popular pop song, drumming on an upturned plastic bucket between his legs. Some passersby stopped, gave him a pitying smile and dropped a few coins into a bowl in front of him.

 

[GoesStation]

Add a comma after smile and they're okay.

 

[Bassim]

GoesStation,
I know you are right. Would you please explain to me why is a comma required in the above sentence? I have seen similar sentences many times, but I don't know what rules applies in the above example .

 

[teechar]

Consider using "he was/started singing" instead of "sang" if you didn't hear the entire song.