All of the "から" parts at least sound natural to me.

> から normally follows the plain form when it means "because"

So, did you think that "(1)...いるから. (2)...きたから", etc? "This also works and just sounds a little more casual.

BTY, から and ので can be interchangeable and may change the politeness and/or nuance as in Kazuko's comment. The general explanation I found says that から is subjective and ので is objective.

Hope this helps.

As a native Japanese speaker, the first half of each sentences sounds natural to me when you would be polite. But each of whole sentences doesn't sound so natural to me. When I say something like them, I will say as follows.

(1)真向かいに二十階のビルが建っていますから、何も見えないでしょう。

(2)あ、雨が降ってきたから、地下鉄に乗りましょう。

(3)私は、中国語が少しできるので、おともしましょうか。

(5) 今週の土曜日に浅草でお祭りがありますから、スミスさんは加藤さんと浅草に行くと思います。or 行くでしょう。

(4) and (6) have no problem.

Sometimes I feel examples in Japanese textbooks for foreign people are too polite to be natural.

Dear both, thank you for your comments, this is very helpful!

@Kazuko: thanks for taking the trouble to rewrite my sentences. Interesting to see the subtle variations. That textbooks are often too polite to make you sound natural was exactly my suspicion.

@Jun: yes, I had in my head that it had to be いる, きた, etc. when coming before から. I understand now that this is not a hard & fast rule but rather a question of style.

I had been afraid that it might be objectively wrong and only to make things "easier" for foreigners - I once even saw an army language course that simply omitted all は、を、が to - supposedly - keep it simple (!) Good to know that this case is different :-).

Best regards,

Aurelio