First thing, you're mixing together levels of politeness. In the first example, you use です which is polite, but then use the short/casual form on 食べる。It should be 食べます。Stick to one level. だから is the same as から, but だから is used only at the beginning of sentences. Think "Therefore, blah blah blah". から is used to connect two clauses, so it's more like our "because" but they put the reason before, not after like in English. Here's how I would correct your sentences (but maybe we'll get a native speaker in here to confirm). 私のともだちは魚が好きですから、私は彼といっしょに魚を食べました。 私のともだちは魚が好きです。だから、私は彼といっしょに魚を食べました。 And now I'll throw in the third option which is what I think you where getting at. You'd be more likely to say this than write it I think, because it would come from forgetting to put the second part in front of the first. 私はともだちといっしょに魚を食べました。彼は魚が好きだからです。