"那么" is commonly used in an assumed situation. Eg, If you had eaten, THEN you wouldn't be hungry. It's quite similar to the use of THEN here, but we know that in English it's not necessary to add "then" to indicate the consequence caused by the first half of the sentence. "如果...那么.." is usually used together but not necessarily. 因为...所以...is just like because... so.... although in English we're not supposed to use both together. 因為１５路公共汽車已經沒有了，所以我們怎麼回家？ ----Because there's no more No.15 buses, so how do we go home? "how to go home" is a question but not a consequence, so it sounds rather weird to use 所以 因为15路公交汽车已经没有了，所以我们要走回家。 In this case, "need to walk home" is an affirmative thing. So, 所以 is better used here. 15路公交汽车已经没有了，我们该怎么回家？This sounds best to me. I know my explanations are lame:(, just hope that it doesn't confuse you more:)

那么 = so, or and then 因为...所以... = because...so...

這兩個句子的語法都對。 但是要看這兩個句子你是對誰說的，所以這兩個句子的意思是不一樣的，因為它適用的場合不一樣。 後面那句可以是反問的意思，著生強調一遍的意思。

那么：I met heron the street and stopped to talk to her。this word show relationship between met and stopped to talk。（not reason and result） we talk ，because we know each other。因为（because）我们认识，所以（so）我们说话。 you know we can’t use two words（because and so）in english。but it is approved in chinese。

我觉得在第一句里，“15路公共汽车已经没有了”是单纯地陈述事实，“那么”是起顺接下半句的作用；第二句的“因为。。。所以。。。”是表示因果关系，强调在“15路公共汽车已经没有了”的前提下，产生了一个问题：“我们怎么回家”。