Questions & Answers about Bayat mayonez yemiş olabilirsin.
How exactly does yemiş olabilirsin translate to 'you might have eaten'?
It combines the verb yemek with the -miş suffix (yemiş) and the possibility form of olmak (olabilir), plus the 'you' personal ending -sin. Literally, it translates to 'you might be in the state of having eaten.' This combination of -miş and olabilir is the standard way to express past possibility in Turkish.
Why is mayonez in its dictionary form here instead of taking the accusative case (mayonezi)?
Because 'stale mayonnaise' is an indefinite direct object here. You are talking about eating some stale mayonnaise in general, not a specific, previously mentioned jar of it. If you meant 'You might have eaten the stale mayonnaise,' you would use the specific accusative case: bayat mayonezi yemiş olabilirsin.
Why doesn't the verb root ye- change to yi- when adding the -miş suffix, like it does in yiyor?
The verb yemek only undergoes vowel narrowing (where the e becomes an i) when followed by a suffix starting with y, such as the present continuous -iyor (yiyor) or the future -ecek (yiyecek). Since -miş starts with an m, the root stays exactly as ye-.