Whoops, I meant to post it as an answer. Sorry! I'm not a teacher, so I can't explain why some of these conjunctions are more suited to the sentence than others. You could rephrase it in a number of ways: He broke his leg by falling from the top of the metal staircase << sounds the most natural. He fell from the top of the metal staircase // and broke his leg // , and he broke his leg // << also sounds natural, but it doesn't denote a 'cause and effect' relationship. It's just stating that both of these things happened He fell from the top of the metal staircase, so he broke his leg << this sounds a bit unnatural (to me). As he fell from the top of the metal staircase, he broke his leg << this also sounds unnatural (to me) [as meaning 'because' in this case, not 'while'] He broke his leg because he fell from the top of the staircase << I would use the first sentence 'He broke his leg by falling from the top of the metal staircase' instead of this. I think that 'so' is used when the first part of the sentence is obviously linked to the second part of the sentence. In other words, the second part of the sentence is a given result. A tumble down the stairs doesn't (usually) result in a broken leg. When 'so' is used to mean 'with the result that,' I believe it can be reworded with 'because.' 'He can speak fluent German because he lived in Germany for 20 years' could be rephrased as 'He lived in Germany for 20 years, so he can speak fluent German.' As he lived there for 20 years, it is likely that he is able to speak German. However, a sentence like 'He fell down the stairs, so he died.' just doesn't sound very normal (to me). Neither does 'He died because he fell down the stairs.' Dying from a fall down the stairs isn't really a given result. I hope this can be of some help.

No, you can't. You must use "thus". Look more clearly at the form of the verb following "thus". You need to pay more attention to such things.

I'm not a teacher, so I can't explain why some of these conjunctions are more suited to the sentence than others. You could rephrase it in a number of ways: He broke his leg by falling from the top of the metal staircase << sounds the most natural. He fell from the top of the metal staircase // and broke his leg // , and he broke his leg // << also sounds natural, but it doesn't denote a 'cause and effect' relationship. It's just stating that both of these things happened He fell from the top of the metal staircase, so he broke his leg << this sounds a bit unnatural (to me). As he fell from the top of the metal staircase, he broke his leg << this also sounds unnatural (to me) [as meaning 'because' in this case, not 'while'] He broke his leg because he fell from the top of the staircase << I would use the first sentence 'He broke his leg by falling from the top of the metal staircase' instead of this I think that 'so' is used when the first part of the sentence is obviously linked to the second part of the sentence. In other words, the second part of the sentence is a given result. A tumble down the stairs doesn't (usually) result in a broken leg. When 'so' is used to mean 'with the result that,' I believe it can be reworded with 'because.' 'He can speak fluent German because he lived in Germany for 20 years' could be rephrased as 'He lived in Germany for 20 years, so he can speak fluent German.' As he lived there for 20 years, it is likely that he is able to speak German. However, a sentence like 'He fell down the stairs, so he died.' just doesn't sound very normal (to me). Neither does 'He died because he fell down the stairs.' Dying from a fall down the stairs isn't really a given result. I hope this can be of some help.

You could use so, as in "He fell from the top of the metal staircase, so he broke his leg." If you are going to change the sentence this way it might be better to say "...and he broke his leg." Or you could say "...consequently breaking his leg."