# if you do these things

The phrase "these things" refers to the things listed in Leviticus 26:14-15.

# I will inflict terror on you

"I will send disasters that will terrify you"

# will drain away your life

"will slowly take away your life" or "will slowly make you die." It is the diseases and the fever that will do this.

# You will plant your seeds in vain

"You will plant your seeds, but you will not get anything from them"