I read somewhere that "used to" usage is for "past habits" and "would" usage is for "past habits and states" and we should only use "used to" for states. Is this true? Suppose that we have this sentences:
Childhood (would)/(used to) be quiet different from what it is today.
People never (would)/(used to) be so worried about crime in this area.
What is correct forms of these sentences and why?
Ps. (would+bare infinitive) vs. (used to+bare infinitive)
In English there are special means of expressing a recurrent or permanent action in the past. They are used to + infinitive and would + infinitive.
Used to + infinitive has only one form — that of the past tense, which occurs in present-time- and past-time contexts. It generally serves to express recurrent actions, which may be either point actions or actions of some duration.
Here are some examples.
I used to meet him sometimes when he was working on the Chronicle here.
I liked reading in the garden. I used to take out a deck-chair, sit under one of the apple-trees and read.
Sometimes used to + infinitive with a durative verb serves to express an action giving a permanent characteristic of the subject of the sentence in the past. In this case, it implies contrast between the past and the present — what was typical of the past is no longer true at present. This meaning is naturally found in present-time contexts.
For example,
“I used to be as sentimental as anyone a few years ago,” said Ann.
Would + infinitive is more restricted in its application than used to + infinitive. It is found only in past-time contexts and serves to express only recurrent actions. On the whole, would + infinitive is typical of literary style.
Here are some examples.
She would often wake up screaming in the night.
He was usually active and interested, but sometimes he would have fits of depression.
Regarding your sentences,
Childhood used to be quiet.
This implies that our present-day life is quite different from it.
People never used to be so worried about crime in this area.
This means that those people are too worried about crime in this area nowadays.
According to Cambridge English Grammar Today, you use 'used to', but not 'would', to describe a state or situation which is no longer true. For example:
We used to live in Manchester.
We cannot use would instead of used to in this sentence.
As both the sentences presented by the OP describe a state, we should use "used to", not would, in them.
