An if statement runs a block of code only when its condition is True:

temperature = int(input("Temperature: "))
if temperature > 30:
    print("It's hot. Drink some water.")
print("Have a good day.")

Three details matter, and Python insists on all of them:

The line starts with if, then a condition.
The line ends with a colon :.
The lines that belong to the if are indented by four spaces. Pressing Tab in the editor does this for you.

The indented lines are the if's block. They run only when the condition is true. The first line that isn't indented is outside the block, so it always runs. Run it again with a temperature of 20 to see the difference.