how/when/where to...
As we know, 'how/when/where to...' is often used as the object of a sentence, for instance, "I don't know how to do it".
Can such a structure be used as a whole sentence, if so, in which cases?
These words and phrases can be used as a sentence. For example: You may say, "How?" It is understood that you are asking, "How can these words and phrases be used as a sentence?", when you simply say, "How?".
Other examples:
"We are going to the park."
"When?"
"Do you want to go for a ride?"
"Where to?"