"Indicates that something has not happened up to the present time."

"अभी तक नहीं"

English Usage: "I have not yet finished my homework."

Hindi Usage: "मैंने अभी तक अपना होमवर्क पूरा नहीं किया है।"

"Up to the present or a specified time."

"अब तक"

English Usage: "Have you eaten yet?"

Hindi Usage: "क्या तुमने अब तक खाना खा लिया है?"