Why yes is not free
Every yes spends hours that were already promised to something else. The colleague who agrees to everything ends up delivering late on the work that counts, and lateness damages a reputation far more than a polite refusal does. People who decline well are usually trusted more, because their yes means the thing will happen. Early in a career, say yes to work that teaches you something or puts you in front of new people, and be careful with work that is merely available.
Before you answer
Do not answer on the spot. "Let me check what I have due this week and come back to you this afternoon" is always acceptable, and it keeps you from agreeing out of reflex. Then find out what is being asked: the deadline, the size of the task, why it came to you, and what happens if it waits.
The shape of a good no
A refusal that works has three parts. It states your constraint in concrete terms, it declines clearly, and it offers something smaller. "I am closing the quarter's books through Friday, so I cannot take the vendor review. I can look over your draft for thirty minutes on Monday, or Sam did the last one and may have the template." Give one reason rather than five, since a pile of excuses sounds like a negotiation waiting to happen. Avoid "maybe" and "I will try," which the other person hears as yes.
When the asker is your boss
You rarely refuse a manager outright. You show the tradeoff and hand back the decision. List what you are working on with the due dates, and say that taking the new item means one of the others moves, then ask which. Most managers have simply lost track of what they already assigned. If the answer is that everything stays and the new thing is added too, say plainly what quality or date is at risk, and confirm it in writing afterward.
What never to decline casually
Requests touching safety, legal compliance or a customer emergency come first, and the rest of the list yields to them. Likewise, do not use "that is not my job" in any form. If a task belongs elsewhere, say who handles it and offer to make the introduction.
Note If you are turning down the same kind of request every week, the problem is structural. Raise the pattern with your manager once, with examples, instead of fighting it one request at a time.
