The mft program creates a TeX file from a Metafont program. It takes appropriate care of typographic details like page layout and the use of indentation, italics, boldface, etc., as illustrated in the book Computer Modern Typefaces. Special conventions in Metafont comments allow you to control things that would not otherwise come out right; section 1 of the MFT source program in the Metafontware report explains these rules.
The command line has one required file name and two optional file names. The required one is a Metafont source file; there is also an optional change file (which works just as the change files to tangle(1) and weave(1) do) and an optional style file (which is prepended to everything).
A file name that doesn't contain a dot is always given an extension, either .mf (Metafont) or .ch (change) or .mft (style). If no style file is specified, the style file plain.mft is automatically used. The change_file_name is not searched for using any paths. The .mf file is searched for using the MFINPUTS environment variable if you have set it, or else the system default. The style file is searched for using the TEXINPUTS environment variable. See tex(1) for the details of the searching.
The output TeX file name is formed by using .tex in place of the extension of mf_file_name.