if / elif / else

Programs get interesting when they choose. The if statement runs a block only when its condition is True:

temperature = 31
if temperature > 30:
    print("It's hot!")
    print("Drink water")
print("(always printed)")

The colon + indentation rule

The : ends the condition line. Every line that belongs to the decision is indented (4 spaces by convention). Dedenting closes the block. Indentation is not decoration in Python — it is the grammar.

if temperature > 30:
    print("hot")        # inside the if
print("done")           # outside — always runs

else and elif

if score >= 50:
    print("pass")
else:
    print("fail")

For many mutually exclusive cases, elif chains read top to bottom and the first true branch wins:

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

Note the order: checking score >= 70 first would give wrong grades — with elif, put the most specific (highest) threshold first.