What are the most important differences between "normal" and auxiliary verbs? Think of the NICE test:
- Both time and modal auxiliary verbs have NEGATIVE forms: hadn't, wouldn't couldn't, isn't, aren't (NOT:
- For questions, the order of auxiliary verbs can be INVERTED: he had = had he?; they could = could they?
- You can have them in shortened forms as long as you know what the CONTEXT is: "They could go there by car, couldn't they?" -- "They could." (The sentence doesn't make any sense if you don't have the question before it.
- You couldn't do this with "You like running, don't you?" --
- They can be stressed with EMPHASIS: "You can't do this." -- "I can!!"
(Source: The Oxford Concise Dictionary of Linguistics, by P. H. Matthews)