Will, Would (willingness, likelihood and certainty)

We use will (or '11) when we talk about WILLINGNESS to do something (e.g. in offers, invitations, requests, and orders) and will not (or won't) when we talk about UNWILLINGNESS to do something (e.g. reluctance, refusal):
• I'll give you another opportunity to get the correct answer.
• Mum! Sue won't give me back my pencil case.
Notice that we can also talk about the refusal of a thing to work in the way it should:
• The top won't come off.
• The key won't fit the lock.
To talk about general or repeated willingness in the past we can sometimes use would, but we can't use would in this way to talk about a particular occasion in the past. Compare:
• Whenever I had to go to town, Ron would give me a lift. (= repeated)
• I was late, so Ron gave me a lift to town, (not ...Ron would give me...) (= particular occasion)
However, we can use would not either when we talk about unwillingness in general or about a particular occasion. Compare:
• We thought that people wouldn't / would buy the book. (= general)
• She wouldn't say what was wrong when I asked, (not ...would say...) (= particular occasion)
We use will (or won't) to indicate that we think a present or future situation is CERTAIN:
• You will know that John and Sheila are engaged. (= you already know)
• 'Shall I ask Sandra?' 'No, don't disturb her - she'll be working.'
• We won't see them again before Christmas.
When we want to indicate that we think a past situation (seen from either a present or futureviewpoint) is certain, we use will (or won't) have + past participle

When we want to indicate that we think an unreal past situation - that is, an imaginary situation or a situation that might have happened in the past, but didn't  - is certain we use would have + past participle:
• I would have been happy to see him, but I didn't have time.
• If your father had still been alive, he would have felt very proud of you today.
• My grandmother wouldn't have approved of the exhibition.