In Word, there are two ways to indent the first line of a paragraph: 1) apply first-line indentation in the paragraph settings; 2) hit the tab key. I often see authors using these interchangeably, switching back and forth within the same manuscript. But what you might not have realized is that, while they look the same in the Word document (unless you turn on the “Show paragraph marks and other hidden formatting symbols” setting), these two methods actually have different effects. Hitting the tab key inserts a tab character, which can be deleted using the backspace or delete key, while using the paragraph settings to apply first-line indentation just tells Word to indent the first line.
The image below shows the difference with “Show paragraph marks…” on and the tab character selected:
Your manuscript will be easier to edit and prepare for typesetting if you’re using one indentation method consistently, ideally the paragraph settings. For a non-indented element such as a block quote, a heading, or the first paragraph of a chapter or section, just change the special indentation to “(none)” and then change it back for the next paragraph.
(Okay, there’s also a third way to indent, which is to type a bunch of spaces at the start of a paragraph. Please don’t ever do that.)