You are asked to take on another task when your week is already full. A long explanation can make the answer sound negotiable even when you have no capacity.
Start with the practical constraint: ‘I can’t take this on alongside the two deadlines already agreed for this week.’ If priorities can change, offer the appropriate decision: ‘If this is more urgent, which existing task should move?’
That question is useful when the requester has authority to set priorities. For a voluntary favour, a simple ‘I won’t be able to help this time’ may be enough. Match the response to the actual relationship.
Do not offer a smaller version of the task unless you genuinely have room for it. ‘I can just have a quick look’ can become another commitment whose scope is never defined.
If you choose to help with a limited part, name the boundary: ‘I can review the first page, but I can’t rewrite the document.’
Before sending, check whether the answer is visible in the first two sentences. The recipient should understand what you can do and what remains unavailable. You can be considerate without creating a defence document for every limit on your time.