Monomials and binomials can be subtracted from each other by subtracting coefficients of terms with the same degree and leaving the variable untouched. Monomials can not be subtracted from other monomials if they do not share the same variable to the same degree.
An example of subtracting a binomial from another is (4*x+8) - (3*x+2). The monomials that contain the variable to the 1st degree can be combined by subtracting 3 from 4, which results in a 1*x term. The single numbers can be combined similarly by subtracting 2 from 8 since both share a variable of x to the zeroth degree. This gives a final expression of 1*x+6.