Most of us decline badly, not because we lack the words but because we are trying to do two things at once: refuse the request and keep the other person comfortable. The second job is not yours.
The shape that works is short. Name the request, give the answer, offer what is actually available.
You are asking me to take the Thursday sessions. I cannot do that this term. I can cover one week in November if that would help.
Notice what is missing. No apology long enough to invite a counter-offer. No explanation that can be argued with. No conditional tense — I do not think I could is an opening, not an answer.
Why the third sentence matters
An unqualified no can read as a door closing on the relationship rather than on the task. One concrete alternative, or none at all if there genuinely is none, keeps the refusal about the work.
You will want to soften it. Read it aloud first and see whether the softening changes the answer. If it does, you have not declined yet.
