Study the words OBLIGE, OBLIGATE, and OBLIGATION:
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If you're obliged to do something, or if you're obligated to do something, that means you must do it: you have to do it because it's important, or because it's a rule or a law, or because it's just the right thing to do: it's fair, it's kind, or it's expected.
Your obligations are the things that you must do.
And, if you oblige someone, that means you do what they asked you to do.
If someone holds out their hand to you, you feel obliged to reach out your hand and shake. You feel obliged (or obligated) to do this because it's polite, and it's expected.
In the show The Walking Dead, zombies have taken over the world, and the few humans left are obliged to form groups and communities. They're obliged to do this because it's the only way to survive: it must be done. They're obliged to band together and protect each other.
If someone tells you, "Please help me eat these cookies—I baked way too many!" then you might say, "I'm happy to oblige," meaning "I'm happy to do what you just told me to do."
Sometimes, "obliged" means "thankful" or "grateful." If someone says "I'm much obliged" or "I'm obliged to you," that's an old-fashioned way of saying "Thank you," or more specifically, "Thank you for how you helped me, and now I'm obliged to help you in return someday."
Finally, an obligation is a job or a task that you must do. Your obligations might include chores, homework, helping to make meals, and taking care of your little siblings. If a friend invites you to a birthday party, but you can't go because you've already agreed to volunteer at the library that same afternoon, you might say, "I'm sorry that I can't come! I have an obligation."