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
Will and would can be used to talk about repeated and habitual behaviour. Would refers to the past.
- When nobody's looking, she'll go into the kitchen and steal biscuits
- On Sundays, when I was a child, we would get up early and go fishing.
A.J.Thomson A.V.Martinet in their book A Practical English Grammar (4th Ed) explain this nicely, better than I could
Habits in the present are normally expressed by the simple present tense; but will + infinitive can be used instead when we wish to emphasize the characteristics of the performer rather than the action performed. It is chiefly used in general statements:
- An Englishman will usually show you the way in the street (It is normal for an Englishman to act in this way.)
This is not a very important use of will, but the past form, would, has a much wider use and can replace used to when we are describing a past routine.
- On Sundays he used to/would get up early and go fishing. He used to/would spend the whole day by the river and in the evening used to/would come home with marvellous stories of the fish he had nearly caught.
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.
- Italian middle schools used to teach Latin. = In Italy, Latin was taught at middle school.
- In Italy middle schools would teach Latin. = same meaning as above
BUT in the negative sense
- Italian middle schools didn't use to teach English. = In the past, English was never taught at middle school.
- Italian middle schools wouldn't teach English. = In the past, Italian middle schools refused/did not want to teach English.
In order for wouldn't to convey the same meaning as didn't use to, the adverb never could be employed.
- Italian middle schools would never teach English.
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.