अब

In a sentence

• अब मैं स्कूल जा रहा हूँ।

  Now I am going to school.

  In this sentence, 'अब' indicates the present moment and signifies the immediate occurrence of the action.

• अब तक उसने अपना काम पूरा नहीं किया है।

  Till now, he has not completed his work.

  Here, 'अब तक' denotes a time span lasting up to the present moment, indicating an expected completion.

• अब आपको धैर्य रखना सीखना चाहिए।

  Now you should learn to be patient.

  In this context, 'अब' is used to prompt someone to adapt a new behavior or quality starting from this moment.