How to fix weird characters in Persian subtitles

What you're seeing is referred to as mojibake. In short, the application you are opening the file with is using the wrong encoding to try and read the file. The standard fix is to use a transcoding tool, either online or offline (though I know of no free ones for Windows which work offline), or open the document in an application that lets you set the encoding and save it through that as the desired encoding.

As a somewhat hacky alternative, if you can save the file without modifying the encoding, you can change the extension to .eml, format it like an email message, make sure the Content-Type header specifies the correct encoding, and then open the resulting file in a good email client (pretty much anything except Outlook or Windows Mail) and copy the text out of there to a text editor and save it.

For future reference, the generally accepted method of avoiding this is to save files as either UTF-8 or UTF-16 (UTF-8 is usually preferred, as it's better supported by most platforms other than Windows than UTF-16).

In particular, your file does indeed appear to be encoded using KOI-8 (determined based on the statement that the text is Cyrillic and the apparent distribution of actual characters), with the application apparently interpreting it as ISO-8859-1 or Windows codepage 1252(determined simply based on what is being displayed, plus the fact that these are standard fallback encodings for many devices).

Some years ago I wrote some notes in a .txt file (plain text) on Notepad and when I opened it recently it appeared with these weird characters. I don't know at what point it turned out like that, but it could have been in-between a transition from a Windows 7 -> External Drive -> Windows 10 (current).

Another .txt files that were in the same folder as this one got like that, although the majority hadn't changed. This makes me suspect of either a conversion error between Microsoft OS or the files got corrupted.

The files got corrupted. It might be a hardware problem or an OS problem, although it's much more likely that they got corrupted when copying from/to the external drive (e.g. via bad USB connection or the drive was damaged) and not during an OS upgrade.

when I opened with Notepad++, this same file was written like this . When I copied it to google translate textbox, the characters with black background turned into some kind of coded matrix, so maybe it can give some kind of clue.

These are "control characters" – they're meant to be interpreted by programs and not shown on screen, and normally they wouldn't occur in a text file at all (except for CR/LF/TAB of course). Therefore they don't have a standard visual representation, and different programs have different ways of displaying them if they do occur:

• Notepad++ (well, its Scintilla core) shows each character's name from the ASCII standard, e.g. byte 0x03 is "EOT" (End-of-Transmission) and 0x18 is "CAN" (Cancel). Some of these names date back to the telegraph era.

• Your browser uses the same method to show all unprintable characters – the 'matrix' is just a four-digit number indicating that character's Unicode codepoint. (In this case they're the U+0018 aka CAN, U+0003 aka EOT, and so on.)

  You can see the same box-with-digits for any character that the OS doesn't have in its fonts, e.g. it will show up for newly released emojis that the OS/browser doesn't yet support.

I already tried to decode through many ways without being successful. Maybe someone has any idea if this is a matter that can be solved through decoding to Plain Text (ASCII), or if the files are corrupted and there is no way back.

In Notepad++, the file looks kind of like it's half-UTF-8 and half-garbage (the accented-'A's tend to show up when an UTF-8 file is misinterpreted as Windows-1252).

However, in this case it's probably just a coincidence and there's likely nothing decodable in this file anymore.