I keep putting in “replacement” and getting it wrong, with the correct answer being “substitution.” I’m not sure there’s much of a difference between these two definitions, that I can see, but maybe I’m missing something subtle? I thought I’d ask before I just throw “replacement” into my custom definitions for 代用
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Not a native English speaker, so I don’t dare comment on the two meanings, but maybe looking at Jisho can give you some further insights on whether this synonym is suitable (I thought the compound words were especially interesting in this case): 代用 - Jisho.org
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The subtle difference in English is that you would replace something if the original not adequate for the taks, while you’d substitute something if the original works, but is unable to be used for some reason.
For example, a player who is dropped from a baseball team for poor performance would be replaced by a better player, while one who is injured during a match would be substituted by an equivalent player. Or, you’d replace a broken light bulb with a working one, while you might substitute a red light bulb for a blue one.
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Thank you for the detailed explanation! I’m always amazed when I realize how much there is still to learn wrt English even 
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I keep thinking and thinking and while we do use substitution for a player in sports, replacement and substitution are synonyms.
We don’t say: hey, could you substitute this lightbulb for this one please? Lol
I’m reading examples online and it is acceptable to use substitute as a verb, however, for the lightbulb example above—as weird as it sounds.
And even as a noun, substitution and replacement are synonyms. They are even listed as such in the dictionary.
I don’t think it’s correct to make the distinction you have made. These 2 words appear to be used interchangeably in the English language. I have certainly never said:
“I’m gonna substitute this lightbulb” because it sounds odd but I think it is correct.
Im fluent in English but I’m not a linguist and don’t have extensive knowledge on grammar rules so perhaps other people could chime in with their opinions. I’m interested in knowing what others think about this.
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I would definitely look up how this is used and what the Jisho.org suggests for “replacement” --there seems to be an aspect of equal exchange in all the “replacement” options in Jisho.org, like replacing a lightbulb, converting currencies, etc. Substitution and its usages don’t seem to have that connotation–if anything, substitute teacher, substitute meat, all seem like a step down, not an equal exchange.
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Right. When we think of ”replace” it’s always with an equal product. Your phone stopped working? They replace it with the same model.
Busted tire? They replace it with a new one. However, we can see that it can be a different brand, it can be a winter tire, summer tire, etc. In this case, the tire may not be the same as the manufacturer but we still say it gets replaced; not substituted.
Just as with your example of substitute teacher, sub meat, etc we could say the same with a car tire, or a windshield wiper I suppose. You’re replacing them even if it ends up being a lower quality. This is bothering me Lol. I mainly use substitution when you want to substitute potatoes for vegetables for example at a restaurant or if you want to substitute a phone case that has the same cost as one made of a different material.
As mentioned, we use replace for something that is broken and you want to replace it with a new one though it doesn’t have to necessarily be broken—it’s just that that’s usually the case. But it’s bothering me because when we use the word “replacement”, the items are not always equal yet we still say it. This means we can just as well use “substitution”.
These sorts of fine, nuanced distinctions are incredibly important when translating literature and poetry.
I’ve no aspirations to become anything other than a rough-and-ready translator, so I wouldn’t hesitate adding “replacement” as a user synonym in this specific instance (to my mind, “substitution” and “replacement” are so close to the same meaning it really isn’t worth maintaining the distinction).
BUT I’ve learned it’s almost always best to just go with the flow and train yourself to respond with the actual primary meaning listed by Wanikani whenever possible. That will almost always be the best single-word choice to associate with that item as a first choice.
Occasionally providing incorrect answers because you didn’t come up with an acceptable synonym won’t cause any harm in the long run. Memorizing an arguably or factually incorrect user synonym will cause misunderstandings eventually.
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For what it’s worth, I agree with @Belthazar’s distinction when talking about the verbs “to substitute” vs “to replace”.
I can’t really think of any situation where I would use replacement and substitute interchangeably despite them being rather similar. For me, I think the distinction, using the noun versions instead of the verbs, boils down to:
A substitution/substitute is used in place of and is often (though not always) temporary. I’d also say it tends to be either an equivalent, or slightly lesser, item. Rarely is it better. It can also be totally different, as in the “red bulb” vs “blue bulb” comparison (though, a replacement can be also, so 
)
A replacement supersedes the original item. It can be an equivalent, or it can be an improvement, but rarely lesser. A replacement is also usually going to be permanent, unless otherwise specified.
In your food example, I think the reason it’s a substitution is because while it’s equivalent (and perhaps in your opinion, a better choice), you’re not replacing the item on the menu, just substituting it for your own plate, so it’s not as strong as a “replacement,” per se. Though it’s an interesting edge case, for sure.
What is interesting, is that English dictionaries do tend to treat “substitution” and “replacement” as synonyms, but I’d honestly be hard-pressed to name a situation where they could be used interchangeably, and it sound natural. This could be a case of, “Well, technically they’re the same thing, and it’s not incorrect to use them in lieu of one another, but it really doesn’t sound right, so it should be avoided in order to sound natural.”
But that’s talking about the English terms of course. I don’t have enough knowledge of the Japanese terms to say whether Japanese would draw a similar distinction or not. I just made sure to learn them as they were taught to avoid mistakes, if that nuance does indeed exist.
Edit: I took out a section and rephrased a bit because I thought about it more and disagreed with myself. 
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