Learn Dutch: Het is simpelweg een stomme fout. - It is simply a stupid mistake.

It turns the adjective simpel (simple) into the emphasizing adverb simpelweg (simply or plainly). While Dutch adjectives often work as adverbs without any spelling changes, adding -weg here specifically emphasizes the undeniable nature of the statement.

Can I attach the -weg suffix to any adjective to make it an adverb, like "-ly" in English?

No, it only works with a fixed, limited set of words to create adverbs of emphasis or manner. Other common examples include kortweg (briefly) and ruwweg (roughly). Never use it as a generic "-ly" translation, or you might end up saying snelweg (highway) when you mean "quickly"!

Could I use gewoon instead of simpelweg here?

Yes, Het is gewoon een stomme fout is perfectly natural and means almost the exact same thing. Gewoon translates more to "just" and is very conversational, whereas simpelweg translates to "simply" and carries slightly more emphasis.

Doesn't stomme mean "mute"? Why is it translated as a "stupid" mistake?

While stom historically means "mute" or "unable to speak" (exactly like the English word "dumb"), its most common everyday meaning in modern Dutch is "stupid" or "annoying." A stomme fout is the standard way to refer to a silly or stupid mistake.