Will fits real, definite futures; would fits hypotheticals and polite framing. In offers both work, with would sounding softer.
Paired examples
- I will call you when the results arrive predicts a real future event. I would call you if the results arrived pictures a situation that may not happen. Same verb, but only one version commits to the call, though would fits if the scenario itself is what you mean to show.
- I will help you move the boxes states a definite offer or promise. I would help you move the boxes, if you asked keeps the help tied to a condition. The change of one word turns a commitment into a possibility.
- Will you join us for coffee? works fine as a direct invitation, and Would you join us for coffee? works in the same slot. Both are acceptable here; would simply sounds a little softer and less pushy, so it is the safer choice with someone you do not know well.
Where the choice hardens
The one place the choice is nearly forced: conditional sentences. A real possibility takes will: If the client approves the budget, we will start Monday. An unreal or unlikely one takes would: If the client approved the budget, we would start Monday.
Notice what changed in that pair: not just the verb, but approves to approved. The past-tense verb signals the situation is imagined, and would pairs with it. That is why would belongs in the second sentence even though the client's budget is a present-day fact.
Conditional sentences force the choice, but everyday requests, offers and predictions leave room for both. In Will you help me move the sofa? and Would you help me move the sofa?, either word works, and politeness usually tips the balance toward would. In predictions, what you know settles it: The client approved the budget, so I will send the draft tomorrow states a definite plan, while I think she would accept the offer hedges on a likelihood. So outside conditionals, let politeness or certainty guide you instead of a rule.