Now, as far as I can see, all the first three types of sentences (i.e. declarative, interrogative, exclamatory) can be without verbs, AKA "nominal sentence". But If I try to find nominal sentence for the 4th type of sentences (imperative), I cannot think how it can be to give an order without verb, or in other words, I'm not sure if it can be "imperative sentence" as a nominal sentence rather than a verbal sentence". What's correct in fact?
According to the linked image in your question, the following are nominal sentences:
- I am curious.
- You are nice.
- She is polite.
- He is our teacher.
- We are students.
Every one of these sentences has a verb. In fact, they all have the same verb, the copular "to be". If we're defining "nominal sentence" as a sentence without a main verb, then none of those examples are nominal. True, this verb lacks action and lacks transitivity, but it does mark tense, form a predicate and demand a subject. The so-called nominal sentence examples from your linked image are all verbal sentences.
English does have a few examples of natural nominal sentences: "Thanks." "Drat." "Sorry." "The more, the merrier." Conversational English and newspaper headlines are rife with contextual ellipsis, leading to a type of technically nominal sentence for which the verb needs to be reconstructed before the clause can be understood.
An imperative is a call to action. Whether explicitly referenced, elided, or merely implied, there must be an action. Certainly, you can make a call to action without a verb. You can make a call to action without a single word at all. A gesture or a glance can make a demand.
Yes but only with the context in a previous sentence that is being ellipsised. E.g. a common one for my kids:
It should be noted that your nominal sentences also include verbs if you consider "be" and its variants (is, are) as such. In this case (equivalently) nominal imperatives would be: "be good", "be quiet" etc