The eight words below can be divided into two groups according to a certain pattern involving two letters. Can you determine the two letters and divide the words into their distinct groups?

STEAL
READ
UNDER
EAR
INK
ROUGH
FOUR
HEAL

HINT:
The two groups are:

READ          STEAL
UNDER       EAR
INK              FOUR
ROUGH       HEAL

Answer:

The two letters that separate the groups are TH. TH can precede four of the words and follow the other four to make new words:

THREAD           STEALTH
THUNDER        EARTH
THINK               FOURTH
THROUGH       HEALTH