This is a little hard to explain, but here goes: "did you remember buying candles for the birthday cake" would be a different question. Here, we want to know whether or not the person remembered that he was supposed to buy some candles, and did he buy them; not whether he remembered the experience of buying them. "I remembered to buy candles" = "i remembered that I needed to buy candles, and I bought them." "I remembered buying candles" = "I remembered the experience of buying candles" or "I remembered that I (already) bought candles" You are right that "remember doing something" where the "doing" happens in the past - but in that case you remember the experience of doing or the fact that you did it. "Remember to do something" means that you remember something that you are supposed to do, and do it. It is a similar usage to "I wanted to do something" or "I tried to do something" I hope that makes sense!

You use to plus verb when you remember things that you have to do or you had to do. You use the -ing form when remember things that happened in the past. In your sentence you have something that you had to do and the correct solution is use to plus verb.

remember to do: remember something and then do it remember doing: something happened in the past and now you remember it. remember is one of those verbs which can be followed by another verb in either the infinitive or 'ing' form. The meaning changes. I remebered to shut the gate. First I remebered to do something then I did it. I remember closing the gate I did it and now I recall doing it.

I mean "the way you remember"

Pay attention to the question and you will get it. It is asking about REMEMBER not about "to buy" or "buying".