One of the major market imperfections are taxes. When corporate taxes exist (and no personal taxes), the situation posited by MM is different.
They proposed that when taxes exist the total value of the firm does change.
This occurs because no matter how well managed is the firm, if it pays taxes, there exists what economists call an externality.
When the firm deducts any expense, the government pays a subsidy for the expense.
It is reflected in less tax.
In particular, this is true for interest payments. The value of the subsidy (the tax saving) is TdD, where the
variables have been defined above.
Hence the value of the firm is increased by the present value of the tax
savings or tax shield.