The phrase "a complex subject that" is correct and usable in written English.
You can use it when introducing a topic that requires detailed explanation or discussion, often in academic or analytical contexts.
Example: "The research paper explores a complex subject that involves various interdisciplinary approaches to understanding climate change."
Alternatives: "a multifaceted topic that" or "an intricate issue that".
Their predictions that the city will sink within 100 years and flood repeatedly involves a forecast of an increase in the sea level because of global warming, a complex subject that Dr. Ammermann has not studied.
When 500 lawmakers do this at once, the result is laws that make "War and Peace" look like a haiku.The Senate's immigration bill covers a complex subject that would be hard to compress too much.
Evolution is a complex subject that requires knowledge of basic biological concepts and the ability to connect them across multiple scales of time, space, and biological organization.
The political situation in the country was obviously very unstable, but as a sports journalist and part of the travelling F1 contingent it's not my job to pass opinion on a complex subject that I am only partially informed on.
One of the main issues is that evolution is a complex subject that requires the ability to integrate knowledge of basic concepts such as DNA, natural selection, and speciation, across multiple scales of time, space, and biological organization (Speth et al. 2009).
"It's like dealing with a professor of a complex subject that you're supposed to know something about," the former chairman of the Securities and Exchange Commission, Arthur Levitt, said of Greenspan when we talked in the fall of 2008.
"The paintings present a very holistic and complex subject that operates in a way that is genuine to the individuals the work is inspired by".
Enrol at a university if you want to get a specialist qualification you can't attain elsewhere, or learn more about some complex subject that you're passionate about.
The story line of "Infectious Greed" is not nearly as compelling, but the book tackles a hugely complex subject that any investor should understand more about.
Economics and climate change are both complex subjects that contain significant areas of uncertainty – especially when it comes to making long-term predictions.
These are complex subjects that resist short explanations, and for which words themselves are often inadequate at best, and misleading at worst.
