I am a volunteer English tutor. My advanced ESL student wrote the following sentence after reading an article:
However, recent studies, in Sweden and in Finland, have found out that different patients suffering from diabetes could be divided into 5 groups, depending on which cluster they suffer from.
Of the four commas in the sentence, I marked commas 2, 3, and 4 as unnecessary. He argued that he was taught that because "in Sweden and in Finland" and "depending on which cluster they suffer from" are non-essential, they need commas around them.
I am more or less certain that the commas around "in Sweden and in Finland" are wrong and pretty sure that the last comma is also unnecessary, but I do not know how to explain how these are essential when non-essential elements, "elements embedded in the sentence that interrupt it without changing the essential meaning" according to Purdue, can be a bit subjective when talking about elements that add more information. Both elements can be removed from the sentence without changing its core (another 'how to determine if it's essential' tip I read), but the commas simply do not look right.
Am I in the wrong? Are these points of information that give the reader more detail non-essential and should actually be surrounded by commas? If not, how do I explain to the student how to determine when additional information is essential or non-essential?