In the name of Allah, the Most Gracious, the Most Merciful,
When do we use the adjective common? Here are several cases in which I would use the word common:
1) When we share something.
In these darkest hours, let us forget our differences and unite to face the common enemy!
2) When something is widespread.
That is a common mistake.
Mat Rempits in Shah Alam? Sadly, that is a common sight.
3) Normal, nothing special.
I am but a common man.
So those are the cases in which I would use the word common. Interestingly, I have also seen the word common prevalently used in such phrases:
Common Manchester United!
This is actually a common mistake. In cases like this, the word common should be replaced with the proper phrase: come on.
"Come on" is used when you want to urge someone to perform better.
For example, during a match, you would say "Come on Manchester United!". In this case, you are urging the football club to play better.
Hopefully this has helped my readers to understand the difference between common and come on.
Allah knows best.