It’s easy to think sentences are independent units of meaning. In a sense, this is true — they express a thought. However, this thought often refers to things that went on before. As a result, the sentence may have references, connotations and emphases that are only clear when presented in context. The context is the dialogue. As such, one way of explaining the differences between similar sentences is to present the different dialogue(s) in which they would most appropriately occur.
For instance, take this pair of sentences:
I mowed the lawn
I did mow the lawn
They are similar and may seem virtually identical to some. Granted, the second one emphasizes the act of mowing — but why use it in preference to the first? The differences (and answer) most clearly emerge if these sentences are presented in their natural environment — a dialogue. Here’s an example (the sentences in question are underlined):
Pat: Did you mow the lawn?
Chris: I mowed the lawn.
Pat: I don’t think you mowed the lawn. Are you sure you mowed it?
Chris: I did mow the lawn.
Now it’s clear. At this point, I would stop (or present more dialogues) and trust students to build their own mental models based on these examples. However, if pressed for an explanation, these dialogues give me the explanation — I can now say the second sentence is a defensive response (although this may not be its sole use!).
Here is another example that came up in class.
I will go to class.
I am going to class.
Someone asked if these sentences were the same. In a sense, they report on the same event and can be used in some overlapping contexts. However, they differ. To best see how they differ, I used a dialogue. By finding the dialogues to which the sentences were the best response, I found and could explain the differences between them.
For the first sentence:
Pat: Would you like to see a movie with me?
Chris: Actually, I can’t. In fact, I’m on my way out right now.
Pat: Where are you going?
Chris: I am going to class.
For the second sentence:
Pat: Your attendance has been poor. If you keep missing class, they’ll drop you. You need to start going to class.
Pat: Sure? So you will go to class?
Chris: I will go to class.
Or take this subtle difference.
I will go to class.
I will go to the class.
Now the first example was already covered by the previous dialogue, so here is the second sentence’s dialogue:
Pat: Your attendance has been poor. Tomorrow’s class is very important — there’s a review for the test. You need to be there!
Chris: I will go to the class.
Now you can see the second sentence refers to a specific class session, while the first refers to attending classes in general. This is key, and it emerges most clearly in the dialog. In fact, while I intuitively know how to use the two forms, this particular usage and explanation only emerged when I used a dialogue. Also note that the two dialogues were intentionally made similar, in order to highlight the differences. By referring to similar events, the students focus on the parts that differ — which are more essential to the response. This is something I use with examples in general, and not just dialogues — but I digress.
In retrospect, a lot of this makes sense. After all, language is most often used in dialogue. People speak, they send messages, they email. They refer to things that went on before and together they share a mental model of a “world” they build. Their sentences refer to this world, and so using those sentences with no reference to the “world” they refer to will inevitably cause confusion.