I have read in Longman English Grammar, page No 246,
that we use 'will' and 'would' to describe natural tendency
Like the simple present tense [> 9.6-8] will (with a 3rd person subject)
can refer to general truths or to the qualities of things; would can
sometimes refer to the past.Water will boil at 100°C It won't boil at under 100°C
Q1: How is this sentence different from: Water boils at 100'C?
I planted a vine last year but it wouldn't grow because it didn't get
enough sun.
Q 2: How is “But it wouldn't grow” different in meaning from “But it refused to grow” and “But it didn't grow”?
Best Answer
Another way of saying natural tendency is calling it proclivity, both will and its past equivalent would are used to express, what Michael Swan (Practical English Usage) describes, habits and characteristics
A.J.Thomson A.V.Martinet in their book A Practical English Grammar (4th Ed) explain this nicely, better than I could
Thus saying that water will boil at 100°C is stating a general characteristic; a natural tendency: the general inclination of water when it is heated at that temperature. Using will in this way is uncommon, but it shows why its past equivalent, would, is often used to express repeated actions in the past. However, would and used to are not always interchangeable.
Examples:
Positive
BUT in the negative sense
In order for wouldn't to convey the same meaning as didn't use to, the adverb never could be employed.
Other examples
As a child I didn't use to wear glasses because I had good eyesight = I didn't wear glasses as a child because my eyesight was good.
As a child I wouldn't wear glasses, although my eyesight was poor. = I refused to wear glasses when I was a child, even if my eyesight was not good. (different meaning)
As a child I would never wear glasses, although my eyesight was poor = similar meaning to 1.