In conversation, we often ask short questions about something that somebody else has just said. There are a number of types.
We often reduce wh-questions in conversation because the speaker and the listener know the context. In the following examples, the short form of the question is more correct, because the full form would sound artificial:
I need to go to the shop.
What for? (full form: What are you going to the shop for?)
We need bread and milk.
I’m going out tonight.
Who with? (full form: Who are you going out with?)
We often use follow-up questions when we are listening, to show that we are interested or surprised. They often do not need a response. They are like response tokens such as really, okay, yeah. Follow-up questions are sometimes called reply questions.
Follow-up questions are formed using the auxiliary verb or modal verb contained in the statement that the question is responding to. If there is no auxiliary verb or modal verb in the statement, we use do in the present and did in the past (the verbs in the statements are underlined):
I left school when I was 14.
It was in the 1950s. Many kids left school early then.