Can you tell me which one is modified and which one that modifies
A good knowledge of three languages is needed for the job
Someone gave this example when she told me that when I have a singular noun that's modified by a plural, the verb that comes after it is singular, but I still don't understand. I also need to know whether that sentence can be written as:
A good three languages knowledge is needed for the job.
Like when we treat plural as singular:
Thirty miles is a long way to run [Oxford Grammar].