Fortran mode assumes that you follow certain conventions that simplify the task of understanding a Fortran program well enough to indent it properly:
Fortran compilers generally ignore whitespace outside of string constants, but Fortran mode does not recognize these keywords if they are not contiguous. Constructs such as ‘else if’ or ‘end do’ are acceptable, but the second word should be on the same line as the first and not on a continuation line.
If you fail to follow these conventions, the indentation commands may indent some lines unaesthetically. However, a correct Fortran program retains its meaning when reindented even if the conventions are not followed.