Let’s break 나무 (the word for tree) up into its parts, taking it out of the square syllable boxes so we can figure out how it's pronounced:
ㄴ
n |
ㅏ
a |
ㅁ m |
ㅜ u |
This word is pronounced na-mu .
And there you have it. It's an application of the rules for syllables:
On the next page are the rules for writing syllables into a square box.