Every accepted request takes space from something else, even when the request sounds small. The cost may be preparation, an interrupted afternoon or another promise now pushed into the evening. Before saying yes, make that trade visible.
Ask what the person needs, when they need it and what adequate completion looks like. A request to look at a workbook could mean a ten-minute opinion or responsibility for repairing the entire reporting process. Agreement without scope lets those meanings drift apart.
Offer a real choice
For example: I can review the input sheet on Thursday and identify the main problems. Rebuilding the workbook would require a separate plan. That answer offers something dependable without pretending the larger job has disappeared.
When two commitments conflict, identify the trade with the person who can decide priority. Which deadline moves? Which part can be reduced? Who else can contribute? Accepting everything and privately hoping to recover the time leaves every stakeholder planning against a fiction.
Close the conversation clearly
A refusal can be brief and respectful. State the limit, offer a suitable alternative if one exists, and avoid inventing excuses that require maintenance later. You do not need to turn every declined request into a negotiation.
Review recent commitments once a week. If work repeatedly expands after acceptance, improve the questions you ask before agreeing. If your limits are routinely ignored, the next move may involve a firmer conversation or outside support. A useful yes is one another person can actually rely on.