Now in many situations, just like my example about the dinner party, 'each' and 'every' can be used interchangeably with just a small difference in meaning, right?
Now, 'speak' and 'talk' are synonyms and most of the time you can use either one of these verbs interchangeably, which means it doesn't matter, you can choose either.
So when we're describing the location of a person or of an animal, we can usually use at and by interchangeably with just a very slight change in meaning.
所以当我们描述一个人或一个动物的位置时,我们可以交替使用 at 和 by,只是在意义上有非小的不同。
There are hundreds of different hydrocarbons in crude oil, and different ones are used to make gasoline and diesel — which is why you can't use them interchangeably.
So, for example, I don't think that being a math person means that you can recite the formulas between the sines, cosines, tangents and to use logarithms and exponentials interchangeably.
Remember that when you look at words which seem to be similar it's important to find out exactly the differences between them because basically you can't really use them interchangeably.
They all mean the same thing, so you can use " Then" as your second step; you can use " After that" as your final step; you can use these interchangeably.