Syntax random

[chelsea]

I'd say "X", but 1) I'm not sure, and 2) I don't have any idea why.

I think I'd expect it to contain X. But I wouldn't be shocked by the opposite.

If you made me choose X or Y, I would say that you probably intend on getting X rather than Y

Not a native speaker, but for me that means "normally there is Y, but I want X instead". Before reading the question and responses, I couldn't even have imagined that some people would have doubts or a different interpretation of this.

The "literal" meaning should be X. substituting something for something means the second thing is what the first thing is standing in for.

For me, "substitute X for Y" is not the most natural formulation, and my immediate guess is that you want Y, which, yes, is technically wrong. Saying "replace X with Y" or using an 'instead of' construction is much more natural. The ordering of the roles of 'substitute' (especially when they're both post-verbal like this) is not very easy for me to quickly retrieve.

Well, I'm questioning reality now. I immediately interpreted it as "use Y in the recipe, not X." I had to spend a minute wracking my brain to see the other interpretation. I agree now that it is unclear. "Substitute X with Y" would be better (to express "use Y, not X")

I reconsidered the structure with people instead ("They substituted Ms. Brown for Mr. Greene") and for some reason, I instantly clocked it as your guys' interpretation. Of course this may just be the previous revelation affecting my judgement.

In case it's relevant, my true first language was Mandarin. I am only a passive speaker now. Still it might be coloring my comprehension..... I did have a /θ ð/ impediment until I was 12. I started learning English at around 3.