Most pages are built from a short list of elements: paragraphs, headings, lists, links, images, and a few containers. Learning that short list covers most writing, and the rest is looked up rather than memorised.
About 1 min of reading
Mark up a short passage.
Write it
Things to buy this week.
Before you run it, say out loud what it will print. Then open the answer.
Add a link
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
A page that jumps from h1 to h4 has a gap a reader can feel and a tool can detect. The level states the depth in the document, so choose the level by where the heading sits, not by how large you want it.
A list of items belongs in ul even when the design wants them side by side. CSS can lay a list out in any direction; the element is what tells a reader there are several things.
p for a paragraph, h1 to h6 for headings in order, ul for a list where order does not matter, ol when it does. Those four cover almost all prose on a page.
An anchor with an href is a link; an img with a src and an alt is an image. Both need their second attribute to be correct: a link without href is not a link, and an image without alt is invisible to a reader who cannot see it.
div groups things for styling; section groups things that are a section of the document. Reaching for a div when a real element exists loses the meaning, so use the fewest containers that carry the structure.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.