"This is too expensive so I'm not buying it."
"因为这个太贵，所以我不买" = Because / since this is too expensive, I am not buying it. "这个太贵，所以我不买" = This is too expensive so I'm not buying it. Fix this English on this, to clear up the confusion.
They accept the translation not using 因為。 Some textbooks also drop the "because" while explaining the Chinese can drop it too.
Alternatively, the 所以 can be dropped instead, but i didn't test that variation.
I initially misread the English as two separate sentences. But the word "so" does seem to connect the phrases in the same way that "because" or "since" would.
Same problem. I have learnt from other sources that if you want to say that something is too something, you have to use 太 something 了, otherwise the meaning changes from 'too' to 'so', which is a big differece.
If I learned one thing from living in China for four years it is that Chinese is a language of brevity. The Chinese simply do not use an excess of words as we do in English. I translated the sentence as follows: "这太贵了，我不买." Yes, its street and colloquial but its real. This is how Chinese people really speak. No need for excess words like 因为 or 所以. Seriously Chinese people try to say the most with the fewest words. It is the ultimate minimalists' language. In Chinese less is more. That's why Chinese love the 成语; only four 汉字！
If 沒買, the sentence should like "it was too expensive so I did not buy it."
Yes. I'm not sure but I think 没 verb is short for 没有 (have not) verb. Anyway, at least it helps me remember what it means.
Although this is a bad way to explain it, that would be more of past tense to use 没. As in to say I didn't buy it or I haven't bought it. Using 不 is more saying, I am not buying it or I won't buy it
I have to use '因为' and '所以'? The sentence didn't say BECAUSE this was too expensive I didn't buy it. Can't I omit the '所以' out right?
It's used that way all the time.
This page has 3 examples with 因为 at the beginning http://dictionary.pinpinchinese.com/definitions/s/%E5%9B%A0%E4%B8%BA-yinwei And this one has 5 https://www.yellowbridge.com/chinese/sentsearch.php?word=%E5%9B%A0%E4%B8%BA