Well if your slider is your header, yes, you should tell HTML that it’s the header.
If you drag a navbar you don’t have an option to say it’s the nav because it’s built-in. But if you create your navbar from scratch, tag it as a nav.
Now for the complete set of rule I’ll let you search on Google, interpretation varies you’ll see… on the “article” tag for example. I tend to follow the rule where you give article to what you can picture being 1 item in a RSS list of your content. So a teaser on a list > article, a full node on a page or a list > article.
<article> element should contain a piece of self-contained content that could be distributed outside the context of the page.
<header> element is used to represent the introductory content to an article or web page. This will usually contain a heading element as well as some metadata that’s relevant to the content, such as the post date of a news article for example. It could also contain a table of contents (within a element) for a longer document.