在科学写作中，下列哪个句子是正确的?

1. The scores were not averaged since we found many outliers.

2. The students said they did not study since they sat the final examination.

3. The teachers last taught in 2000 and did not teach since then.

4. The teachers said they taught since 30 years.

这四个句子都可以改进。“Since”是一个从属连词，意思是与过去的原因/理由或时间有关。在句子（1）中，“since”真正的意思是for the reason that而不是from the time that，所以句子使用逗号和“because”会更清晰：“The scores were not averaged, because we found many outliers.”

句子（2）有歧义。如果“Since”的意思是from the time that，那么动词“study”应该是过去完成时态，以表明studying在过去的某个时间点停止了：“The students said they had not studied since they sat the final examination.”但是，如果“Since”的意思是because，最后一个子句应该用过去完成时来表明原因是一个先前的事件，也应该加上副词“already”：“The students said they did not study, because they had already sat the final examination.”在这句话里，“did not study”暗示not studying是一个新的习惯（相当于“were not studying”）。

注意，修改后的句子（1）和句子（2）中在“because”前面有一个逗号，用来告诉读者不做某个动作的原因。如果没有逗号意思就变成：你在解释你为什么做这个动作，但是否定了一种可能性，读者会期待接下来的真正原因（“The scores were notaveraged because we found many outliers; they were averaged because…”）。

在句子（3）中，“Since”是一个介词，意思是from a specific past time（自从一个具体的过去时间以来）。在这句话里要使用过去完成时：“The teachers last taught in 2000 and had not taught since then.”“Since”也是一个副词，可以表示从某个具体的过去时间到现在，所以另一个修改的选择是“The teachers last taught in 2000 and had not teaching Since.”但是，在特定的时间点之前加上介词可以使句子更加简洁：“The teachers had not taught since 2000.” 

句子（4）中没有时间点，只有一个时间段。因此，正确的介词是“for”，需要使用过去完成时态：“The teachers said they had taught for 30 years.”