# sign for you

"sign for you, Hezekiah." Here "you" is singular and refers to Hezekiah.

# you will eat ... you must plant

Here "you" is plural and refers to the people of Judah. 

# in the second year what grows

"in the second year you will eat what grows" or "next year you will eat what grows" 

# in the third year

"the year after that" or "in the following year"