After having discussed the overall Structure of Papers, it is now time to look into the different parts of the paper structure, and see if there are more patterns we need to be aware of (spoiler: there are).
We said about the purpose of a paper:
The main purpose of a paper is to convey a new idea, and show that it has merit.
Then, we can say about the role of the introduction:
The main purpose of the introduction is to identify a problem worth solving, and to show that you are indeed able to do so.
The shape of the introduction is much more formalized than any other part of a paper, which helps and guides us writing one. In particular, we find
State the "BIG Problem" your paper addresses (even if only ever very indirectly). Start out with some BIG Problem (TM), that everyone can relate to. Climate change? Great. COVID? You got it. Gun violence, polarization of the society, etc. 1-2 sentences.
Your topic, hyperspecific. Guide the reader from the BIG Problem statement to your particular hyperspecific research question. You want to solve COVID? Did you know that this one particular protein is superimportant? You want to solve global warming? Did you know that the molecular interaction of carbon dioxide at the critical point is super relevant? 1-2 sentences.
Literature review. This is a tricky one: you have to give the reader an overview, but also demonstrate that you talk the talk and walk the walk and know the lingo of your domain. The meat of the introduction. Describe what is known, what others (and maybe yourself!) have done, what the current state of knowledge with regards to your hyperspecific topic is. 1-3 pages.
What isn't there? The Knowledge Gap! Clearly identify one aspect that no one has addressed yet, something that isn't known yet, some open question. 1 paragraph.
How are you going to fill that gap? Your Research Goal. State it very clearly, "The goal of this paper is to xxx". 1 paragraph.
Some like to give an outline of the paper in that last paragraph, me personally, I think it's a waste of space. Why? It's always the same. Always: The Structure of Papers.