Can these two words (secretly and secretively) be used interchangeably?
- He fed the dog secretly.
- He fed the dog secretively.
Which of the two sentences above sounds more appropriate?
Secretly means that he did it in secret, likely no one saw it because it is still a secret.
Secretively means that he did it in a way that suggested that he wanted to keep it secret. Someone might have seen him though.
The comment by FumbleFingers and the answer by 米凯乐 are good, I just want to add a little more color to it.
"secretively" is a description of what the action looked like when it was done, and "in secret" or "secretly" is a statement of fact about whether anyone else knows he was feeding the dog.
I think the sentence "He fed the dog secretively." would most likely be said as a description of the behavior. The person might be somewhere that it was inappropriate to be feeding one's pet, so he did it in such a way as to conceal the activity from nearby onlookers.
"He fed the dog secretly", which I would agree means the same as "he fed the dog in secret" would be more like something stated from the point of view of "omniscient story teller", or stating the facts of a case.
And if someone was feeding a dog in secret, many of their actions relating to it might be done secretively since they are trying to keep the fact that they are feeding the dog a secret. So there is a potential overlap of meaning there in some sense.