I am a first year student at a UK university and submitted an essay for one of the courses a couple of months ago. Now, I received an email in which I am informed that some passages of my essay are too similar to the sources, and that the sources were not satisfactorily referenced. I have never worked by a reference system before, because my country doesn't use any. I have 10 days to write them back, and if I don't, a formal investigation will be carried against me for possible academic misconduct. What should I say in the written statement and is my lack of experience a good enough motive?
If I understand this correctly, you were under the impression that proper quotation of sources means to have them in your bibliography (and nothing else). You then quoted from those sources without quotation marks and footnotes to the reference quoted.
I’d explain that to them, apologize, and ask for a good source how, in the future, to quote according to their specific guidelines. That’s the truth; it doesn’t strike me as terrible in your first year, but you’ll have to hope your professors agree.
I would consider replying that, since you weren't writing for publication and you were using course provided resources, you didn't recognize the need, at least partly due to inexperience with such things.
But make it part of an apology, I think, and a promise to take better care in the future.
If there were clear instructions about citing given for the assignment or the course then such an appeal will fail, of course.
But (opinion), instructors need to make the standard clear. I don't think it is fair to expect first year university students to understand all of the conventions of quoting and citations without instruction. We aren't born with a "citation" gene, for example. If that instruction was provided, then you are probably lost. A "slap on the wrist" might be the best possible outcome. Live and learn.
I'll also note that the writer (yourself) and the reader (the instructor) have a common understanding of the base ideas and sources, making citation less of an issue. People seldom cite their textbooks, for example, treating the ideas there as common knowledge. But you can't attribute the ideas of others to yourself. That goes beyond copying or paraphrasing.
Since you are a first-year student, the bar here is pretty low. Still, your professor has threatened to escalate this to a formal academic misconduct matter (presumably based on an allegation of plagiarism) and so you should note the standard Miranda advice used in policing: anything you say may be used against you. In view of this, you should immediately read the academic misconduct procedure at your university, and check the rules for when you are required to make a statement in your defence. At virtually every university in the Western world, due process requirements mean that you are not required to respond to an allegation until after you are given a written statement of the details of the allegation in the formal proceeding.
In the present case, since you have been told that this may be escalated to a formal matter, I recommend that you do not address the allegation at all. You should write back politely and say that you are open to talking to your professor to learn more about how to cite sources properly, but you do not propose to comment on your submission, since it is now anticipated that this may lead to a proceeding for academic misconduct.
(As a final note, I think you are almost certainly wrong when you say that your country doesn't use any referencing system. Academic work in virtually every country in the world abides by some conventions and rules on referencing. Perhaps the guys in Hutt River Province don't, but virtually every country does.)