I don't believe in English you could say 'a plane mirror' as in a flat mirror. In English 'plane' does mean 'flat' but it wouldn't be used as a describing word. 'A plane', means a large flat landscape, or to 'plane off a piece of wood' (make it flat). You wouldn't use it as an adjective. You can either say 'a flat mirror' or 'a plain mirror' - but 'plain' means unadorned.
Well, after a bit of research i found out that plane and plain both refer to flat area, but the difference which appears among them is that plane is a geometrical term where it is used to describe a two dimensional area for example " a flat table", and on the other side "Plain" is a geographical term for example " a plain valley" or "a plain land". Both words mean the same, unless it depends where its being used.
Although you have their origins established, "plain" is an adjective to describe something as ordinary, unremarkable, or unadorned with any ornaments. This may come from the fact that plains (which are also planes) are usually plain because they lack any remarkable geographic features aside from their plainness. But this is just a guess, so i recommend not relying on that as anything other than a mnemonic device.
"Plain" has a very different meaning from "plane" in most contexts, though they share numerous peculiar similarities and share a common root. You're technically correct, but I just wanted to clarify on their modern usage.
Plano (as noun) = plane (1. the geometry term for a flat region of space, or 2. a tool for flattening a surface—not the shortened form of airplane). Plano (as adjective) = planar (the adjective form of plane #1; pronounced exactly like "plainer") or its more popular Germanic synonym: flat.
This does not make sense. I can't remember ever hearing someone use the phrase "plane mirror" in English. "Plain mirror" was my answer, and it rejected it. Plain would make sense because many mirrors are very ornate and decorative, whereas some are simply a square reflective surface.
The word meanings for espejo were mirror and model. The meanings for plano were flat, level, and plane. So why isn't "I have a model plane" correct? Seeing this sentence, or hearing someone say it, how would I know they were talking about a flat mirror and not a model plane, or level mirror, or model flat?
Wrong use of "plane"; here it means a flat 2-D surface as found in a 3-D universe, not a flying, winged vehicle. However, the true translation should be the adjective "planar", which means in the shape of a plane, i.e. flat. Why Duolingo doesn't suggest "planar" is beyond me.