Turn nouns into plurals
I'll start us out with something simple.

Javascript 232+2730=2962

w=prompt();r="ay,ays;ey,eys;oy,oys;y,ies;us,i;um,a;is,es;x,xes;man,men;s,ses;sh,shes;ch,ches;z,zes".split(";");for(i=0;i&lt;r.length;i++){b=r[i].split(",");if(w.slice(-b[0].length)==b[0])return w.slice(0,-b[0].length)+b[1];}alert(w+"s")


Uses a small list of replacement rules. Fails 2730 test cases out of 95642.