"Yes, I had already supposed it."

Translation: Sí, ya me lo había supuesto.

[claypearson]

Why is the "me" required?

[LBlendDuolingo]

I don't know why they used "me" here but you can use "yo" instead and it would still be correct

[Iris150201]

I used "yo" and it was still wrong

[doublelingot]

'Yo' and 'me' aren't necessary. Sí, ya lo había supuesto.

[Haengcha]

What's wrong with "Si, yo ya lo habia supuesto"

[DaveHarris809825]

This construction occurs elsewhere in this section of the tree. See

https://www.duolingo.com/comment/650688/I-had-already-drunk-a-coffee

Rspreng says:

Había can be I, he, she, it. The "me" shows which is the subject.

There is an alternative explanation offered by some people which focusses on the way in which many verbs such as 'tomar' and 'suponer' change meaning in the reflexive.

Still others would use "yo" instead of "me" or omit it altogether.

I have a feeing this may be a regional thing. I will try to ask a couple of local speakers here in Gran Canaria and add to this comment if I find anything definitive.

[Iris150201]

Is there a simple clue to alert learners that a sentence is reflexive? What does one look for?

[Linabuela]

How does one determine the word order of si, ya me lo

[Brycecrab]

Ya is generally at the beginning of a sentence while me is generally just before the verb.

[DABurnside]

Please see https://forum.wordreference.com/threads/suponerse.1993921/

This site says: suponer can mean suppose, imagine, mean (imply), whereas suponerse can mean assume, imagine, suppose. The site gives examples that are helpful.

[Hokiegb]

I did the same thing, "Si, yo ya lo habia supuesto" and marked wrong. If anyone knows why this is reflexive, I'd love to hear it.

[mfe607860]

Strange enough, my 'Si, ya lo había supuesto' was accepted May 31, 2020.
Is the reflexive use optional ?