Use an example and input these words into it. Say, the sun came out so the the flowers bloomed. The sun came out <em>if </em>the flowers came out? No. This could be used to say, if the sun came out the flowers would bloom. But, this isn't a effect sentence. The sun came out <em>since </em>the flowers came out? No. The sun came out because the flowers bloomed? Also no. This would work if you where signaling a cause, the flowers bloomed <em>because </em>the sun came out. So, this leaves therefore. The sun came out, and therefore the flowers bloomed.
