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if/elsif/else/end. The then keyword is optional. Ruby also has unless as a positive way to write if not:

if x > 0
    puts "positive"
elsif x < 0
    puts "negative"
else
    puts "zero"
end

puts "go!" unless ready_to_quit

Any if/unless can be appended as a modifier — useful for short guard statements:

puts "too low" if score < 60
return unless valid?

case/when is Ruby's pattern-match-y conditional. The when clauses use ===, which makes ranges and classes work naturally:

case score
when 90..100 then 'A'
when 80...90 then 'B'
else 'F'
end

In Ruby, only false and nil are falsy. EVERYTHING else (including 0, '', empty array) is truthy.