I'm making subtitles on a film that's primarily interviews of Spanish-speaking parents with subtitles. I'm just wondering what people do for grammar on screen - do you always include periods and commas the way you would if you were writing on paper? Are there any conventions regarding leaving them off for film?
You do whatever is needed to bring the message across, in a areadable form.
Most of the time, you need to shorten the text.
This always means consessions. You cannot have things like double negatives, as it takes too much screensize / words and is way too confusing.
But this also means you include any profanity, translated to your language. (That's a form of art by itself).
Also, local differences have to be translated. (Some cultures never use the word 'no', but intend it as much as we do. Up to you to decide...)
Keep in mind that people watch the image, and just glance at the subs.
Rule of thumb, if you can read a title out loud in the time it's on screen, you're save.
(If it is easy to understand text.)