"No, I did not go to the shop on Wednesday."
Translation:Naddo, es i ddim i'r siop ddydd Mercher.
This is now explained in the notes for this section, Past 2.
As has been mentioned in previous notes, in Welsh for the positive and negative response to a question there is not simple 'Yes' and 'No', we affirm or negate the question.
Usually the answer depends on the form of the question eg 'Dych chi'n mynd?' (Are you going) ; 'Ydw' (Yes, I am); 'Ydy hi'n mynd?' (Is she going?); Nac ydy (No she is not)
In the past tense ( and also the perfect tense in North Wales) it is much easier.
The answers, whatever the form of the question, are Do (Yes) and Naddo (No).
To answer a question in the past tense you use "Do" and "Naddo". The "Ydw", "Wyt", "ydy" etc forms are used to answer questions that start with a present tense form of "Bod". Otherwise you simply repeat the verb relevant to the person e.g "Hoffet ti ddod gyda ni?" "Hoffwn" etc.
It should be explained in the Notes for the part of the course dealing with the simple past tense.
Was I correct to report "Naddo, mi es i ddim i'r siop ddydd Mercher" as a correct answer? Can "mi" be used in a negative sentence?
No, mi is only used with positive statements.
Generally, you will only come across it regularly in parts of mid and north Wales (as with fe being used in a similar way in parts of south and south-west Wales), so unless you are living in or regularly visiting those parts of Wales it is probably easiest to be aware of it but to leave it out of your own Welsh.
As Ibisc said it can't and the reason is because in traditional Welsh the "Mi" would become a "Ni" in negative sentences (which is why we always mutate at the start of a negative sentence). Though note you can't use "Ni" and "Ddim" at the same time.