Use of そ and あ to refer back

Words beginning with そ and あ (and occasionally こ) are used to refer back to previously mentioned topics and phrases. 

きょ年、日本に　行きました。そのとき　はじめて　ラーメンを　食べました。　

(I went to Japan last year. It was at that time that I first ate ramen noodles.)

たん生日に　本を　もらいました。それは　日本の　ベストセラーしょうせつ　です。

(I received a book for my birthday. It's a best-selling novel in Japan.)

Words in the あ group tend to refer to some information shared between the speaker and the listener.

もう「ホットスポット」みた？　うん、あれはおもしろかったね。

Have you already watched 'The Hot Spot?'    Yes, that was interesting, wasn't it?

When the information is new to the listeners, use そ group.

もう「ホットスポット」みた？　ううん。それ何？　ドラマ？

Have you already watched 'The Hot Spot?'  　No. What's that? Is that a drama?

私は　さいたま　という町に　すんでいました。そこには　大きい　じんじゃや、　ゆうめいな　はくぶつかんが　あります。

I lived in a town called Saitama. There is a large shrine and a famous museum, among other attractions.