Can we use hugo commands in markdown files?

Agreed; it’s not the strength of any generator to really focus on single-page websites (note: although SPAs are a different story). That said, I actually thought this was the original thought behind .GetPage (you’ve already corrected me, @bep.

[quote=“mfima, post:5, topic:4937”]
I would like to have something like “{{% insertmarkdownfile path/to/somefile.md %}}” to put, as part of the content of just a few pages, or maybe just ONE page, and those alone, one block of text that is common to them all, but:
[/quote])

If the content is going to remain the same across multiple pages, I’d recommend a partial with HTML. This would give you the added benefit of dynamic values you can pull from the front matter and context.

If you’re talking about reusing the same block in different spots within content, perhaps a shortcode that just pulls in that HTML directly into where you want it on the page. If you want the added convenience of additional markdown within the shortcode, use the {{%%}} syntax.