used to introduce a conditional clause meaning "except if"

"अगर नहीं"

English Usage: "You won't succeed unless you work hard."

Hindi Usage: "आप सफल नहीं होंगे अगर नहीं मेहनत करेंगे।"

indicates a condition that must not be the case for the rest of the statement to be true

"जब तक"

English Usage: "I won't go unless you come with me."

Hindi Usage: "मैं नहीं जाऊँगा जब तक तुम मेरे साथ नहीं चलोगे।"