Generally To play movies with subtitle from USB connected to Samsung TV, you have to download the subtitle file (from example http://www.subscene.com/) rename the subtitle file name to be the same (with different extension of course)  name as your movie name and you need to select Samsung profile in the console.
the thought related to the title here is that some subtitle  files with .SRT extension  are not recognized by the TV, this is usually because the timing sequence beginning with 0 sequence number instead of one (inside the srt file).
To fix this all you need is to remove the line that starts with 0 .