I see it less as a matter of certainty and more of a matter of situation.
Your first sentence is dealing with a hypothetical situation.
I wouldn't sell my house for two million dollars.
That means that, even if someone were to offer me two million dollars for my house, I wouldn't accept the offer. No one is making such an offer, but I'm saying that I wouldn't accept it.
The version with "won't" may be talking about something more concrete.
I won't sell my house for two million dollars.
The tone of this sentence makes is sounds like a real offer is either in the works or on the table, and the person is saying they will not accept it.
Going back to your sentences:
1) My daily time with Sandra is something I wouldn't trade for anything.
2) My daily time with Sandra is something I won't trade for anything.
Both sentences mean that you value your time with Sarah, but in the second, it sounds like someone is trying to get you to give up that time – perhaps by rearranging your schedule – and you don't plan to budge. But I wouldn't regard the first sentence as "less certain."
(1) and (2) are conditional sentences, not conditional phrases. Conditional sentences are formed by a conditional clause (in this case, "If I found out the truth / If I did find out the truth") and a main clause (in this case, "I would definitely tell you").
Both sentences are slightly different in that the verb in conditional clause (2) is emphatic, so we can imagine some preceding context in which the possibility of "I" discovering the truth is considered to be remote, for example:
- I don't think I will ever find out the truth, but if I did find out the truth, I'd definitely tell you.
Best Answer
When 'already' is used with negation, it most likely emphasizes that something should have happened by the moment.
'Not yet' is a neutral expression for something expected to happen soon.
So in your first example the teacher probably planned to cover that topic by the moment, in the second one the time for the teacher's explanation has not come yet.