Professor: Now when I mention the terms "boom and bust", what does that bring to mind?
Student: The dot-com crash of the 90s.
Professor: OK. The boom in the late 1990s when all those new Internet companies sprang up and were then sold for huge amounts of money.
from TOFEL TPO6 Listening
This is a part of a lecture in an economics class.
The subject is 'The boom', but I can't find a verb. why?
Can I use this sentence in writing? Or is it only used in speaking?
The "sentence" in question is grammatically just a noun phrase with no verb attached to it.
In this context, however, it's correct, just as a noun or noun phrase is in this context:
Alice: Where are we going for lunch?
Bob: The Mexican place where the servers wear wrestling masks. Alice: The same place we went yesterday.
Here, Alice asks a question, and the next two lines of dialogue are just noun phrases, not sentences, because in a dialogue, a noun phrase is a grammatically correct answer to a wh-question.
This is almost only used in speaking. A writer who uses this style creates a conversational tone, usually with an imagined reader.
