Used to ask about the time something happens.

जब

English Usage: When will you arrive?

Hindi Usage: तुम कब आओगे?

Used to refer to a specific time or condition during which something occurs.

जब

English Usage: I remember the day when we first met.

Hindi Usage: मुझे वह दिन याद है जब हम पहली बार मिले थे।

Used to refer to an unspecified time.

कब

English Usage: Do you remember when?

Hindi Usage: क्या तुम्हें कब याद है?