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Here is one verse I don't understand that much, because the verb to be is in singular form, and the subject is plural. Also, what is the exact meaning of bustling hustling their heart out ?

If I'd like to write it as a declarative sentence, where should I put commas?

'' Streets is bustling hustling their heart out ''

Thank you a lot !
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These are rap lyrics. Songs in general are almost free of grammar constraints, and rap music in particular is coloured by ebonics as well as the freedom to subordinate grammar to rhythm and rhyme. These following lines all have grammatical or other anomalies:

Picture the morning taste and devour
We rise early pace up the hour
Streets is bustling hustling their heart out
...
Spices herbs the sweet scent of flower
We came out precisely the hour
...
I had dreams beyond our border.

...
And why do they say that the children have rights to be free
...
How come you ain't come up the ladder
...
You so bright, you shine like my TV

Comments  
You have not posted a verse; you have just posted a single line. Please post more of the context and tell us its source.
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Sure, here is the verse.

Picture the morning taste and devour

We rise early pace up the hour

Streets is bustling hustling their heart out

You can't have the sweet with no sour

Available on: http://www.songlyrics.com/k-naan/fatima-lyrics /
 Mister Micawber's reply was promoted to an answer.
Thank you for your answer!

Yes, it seems that this types of songs are not the best way to learn grammar, but would you please tell me the meaning of: streets is bustling hustling their heart out. Thank you in advance.
Students: We have free audio pronunciation exercises.
The streets are very busy.