You can burn text subtitles (hardsubs) with one of two filters: subtitles or ass.
Draw subtitles on top of input video using the libass library. This filter requires ffmpeg to be compiled with --enable-libass. See the subtitles video filter documentation for more details.
If the subtitle is a separate file called subtitle.srt, you can use this command:
ffmpeg -i video.avi -vf subtitles=subtitle.srt out.avi
If the subtitle is embedded in the container video.mkv, you can do this:
ffmpeg -i video.mkv -vf subtitles=video.mkv out.avi
Same as the subtitles filter, except that it doesn’t require libavcodec and libavformat to work. This filter requires ffmpeg to be compiled with --enable-libass. On the other hand, it is limited to ASS (Advanced Substation Alpha) subtitles files. See the ass video filter documentation for more details.
ffmpeg -i video.avi -vf "ass=subtitle.ass" out.avi
If your subtitle is in SubRip, MicroDVD or any other supported text subtitles, you have to convert it to ASS before using this filter:
ffmpeg -i subtitle.srt subtitle.ass
Windows users will have to setup font paths to get libass to work
You can burn "picture-based" subtitles into a movie as well, by using the overlay video filter to overlay the images. For instance, dvdsub is a type of picture-based overlay subtitles. Example of an MKV with dvdsub subtitles in a separate stream:
ffmpeg -i input.mkv -filter_complex "[0:v][0:s]overlay[v]" -map "[v]" -map 0:a <output options> output.mkv
If you have multiple subtitle streams, you can select which one to use by replacing [0:s] with [0:s:0] to select the first subtitle stream or [0:s:1] to select the second subtitle stream, and so on. See also the official documentation; search for "hardcode".
Sometimes adding this filter have a side effect of breaking encoding when input file has multiple audio streams and some of those may have problems; in this case selecting one (or more) of the audio streams should fix the problem:
ffmpeg -i input.ts -filter_complex "[0:v][0:s]overlay[v]" -map "[v]" -map 0:a:0 <output options> output.mkv
Note that on windows you may have to take added measures to setup your fontconfig appropriately, etc: https://ffmpeg.zeranoe.com/forum/viewtopic.php?f=7&t=2554