itは名詞が単数または数えられない名詞のとき、
them、theyは名詞が複数の時、someはその中でもいくつかを表す
oneは基本的に名詞そのものでなく、同様のものをあらわす時に使います。

■itとthat,this
this, thatは現実界で距離感を伴う、itは文中の言い換えで一般論化を伴う

※this, thatも文中の言い換えを行う
itは一般論化が強いので、特に会話の流れではthatを良く使う
To be or not to be : That is the question.
We see him taking a walk, but this isn't very often.

it > that > this 抽象度で使い分ける
I will work on it.　（距離感が遠いし現実感も少ない）
I will work on that.（距離感が遠いが現実感がある）
I will work on this.（距離感が近いが現実感がある）

■It
the 単数
What is this? It is a book.(This is a book.)
不可算名詞
Watar is very useful. We use it to generate electricity.

■them, they
(代名詞を使わず、生の名詞を使うことが多い？)
the 複数名詞
Have you ever seen these pics? Yes, I saw them some days ago.
Truffles are delicious, but they are too expensive.
I saw those pics.

■some
I saw some.

■one
数えられる名詞を受ける - one
Jack bought a nice bike. I want to buy one, too.
形容詞＋普通名詞 - a adj one(s)
Nancy bought an expensive camera, but I bought a cheap one.
Sorry, we have only brown ones.
特定のものを表す - the xxx one
I'd like the smaller one.
世間一般の人
One should do one's duty.

■全然無い - there is(are) none
Are there any guests in the living room? No, there are none.
強い否定
None of her work has been done.

■Both, All の否定は部分否定
He didn't invite all the people.
彼は全ての人を招待したわけではない
She did not throw both of them away.
彼女は両方を捨てたわけではない

■Either, Neither
Either(両方もしくはどちらか一方)とNeither(両方ちがう)
I don't like either of them. = I like neither of them.