Subtracting from a specific column

You could use: MapAt:

MapAt[ # - 1 &, list, {All, 1}]

{{-1 + a, b, c, d}, {-1 + e, f, g, h}, {-1 + i, j, k, l}}

or Apply at the first level (shorthand @@@) (see also SlotSequence, shorthand ##n):

{#1 - 1, ##2}& @@@ list

This is quite neat:

list[[;; , 1]]--

If you write

list[[All, 1]] = list[[All, 1]] - 1

list will be updated with the new values of the first element of each sublist.

Related