There are now two forms of the source code. The old source code form, which is based on the punched card, and now called fixed form and the new free form. In the same program we can mix program units written in fixed form and free form, but each unit must be only in one form and at the compilation both forms may not usually be in the same source file. Some systems using a special directive may permit program units in different form in the same file. The directive then tells the compiler which form is valid.
Some old statements are to be avoided.
It is possible to mix old and new statements, but it is advised to try to be consistent, which means to use either the old or the new form for the statement. Note that when you switch from one compiler to another new errors may occur, because the old compiler was not as strict as the new one. Normally, a new compiler discovers some old errors that were not found earlier. In the specification for Fortran 90 it is required that errors are to be found already at compilation if this is at all possible. Get some instructions in Appendix 6, NAG's Fortran 90 or Absoft's Fortran 90.