1. The same change can turn these three words into three new ones. What is it?

NET
SEES
FLEES

2. The same change can turn these four words into four new ones. What is it?

PINK
BOOK
NIGHT
BROWN

Answers:

1. The letter X can be inserted into each word:

NEXT
SEXES
FLEXES

2. The letters IE can be added to each word:

PINKIE
BOOKIE
NIGHTIE
BROWNIE

Credits:
#1: Jack McInturff
      ELIMINATING THE UNKNOWNS

#2: Joel D. Lafargue
      THAT IS TO SAY