Playing DVDs with External Subtitle files
First let me begin by saying that this is a rather technical somewhat complex subject. It's not for the new user.
Most DVD discs contain internal subtitle streams, this article is about external subtitle streams stored on files that are not part of the original DVD.
In order to enable external subtitle support in Zoom Player, you must have DirectVobSub (VSFilter) installed. You must next instruct Zoom Player to use this filter in DVD Mode. This is done by going to "Advanced Options" and opening the "Playback / DVD - Smart" dialog and under the "Additional Filters" section, adding "DirectVobSub".
There are several ways to load the external subtitle files when the DVD is played. The Manual approach is to play the DVD and once the movie itself starts playing, press Alt+"S" and using the DirectVobSub settings dialog that will pop-up, browse the subtitle file you want to play.
The Automatic approach is to place the subtitle file in the DVD's bookmark folder with the base file name (file name without extension) being "disc". For example "disc.sub" if your subtitle format is ".sub". The DVD bookmark folder will only get created after you play the disc once. If your DVD is backed up on a local hard drive, you may place the subtitle file within the same folder as the VIDEO_TS.IFO file.
Please note, DVDs were never meant to be played with external subtitle files. You may face several issues when trying to get this to work. These issues include no video image at all, subtitles never showing up or subtitles going out of synchronization.
For no-video image you can try switching between the video renderer options to see if a different renderer makes a difference, for no-subtitles try opening the DirectVobSub settings page (Alt+"S") to see if a subtitle file is actually loaded (it's quite possible that some subtitle formats will not work with DVD Playback!). If your subtitle goes out of synch (or even starts out of synch), you can try using the DirectVobSub settings Page (Alt+"S") to compensate for the skew.
Once again, there is a good possibility this won't work at all, YMMV.