Dear teachers of English native speakers:
I have got a question here. Please give me your opinions.
If I wanna express the meaning "When they come back , the dishes will still be warm", can I say: "The dishes will keep warm until (till ) they come back "? What about :The dishes will keep warm before they come back?
Your original statement was perfectly adequate: "When they come back, the dishes will still be warm" although I would probably have reversed the phrase order: "The dishes will still be warm when they come back."
"The dishes will keep warm" is awkward. I'd not use that. Maybe "stay warm" might work better. But your first, "still be warm" was the best.
The "warm before they come back" won't work at all. That says that the dishes are electric and they will warm (heat up) before they come back. It makes "warm" a verb, when you actually mean to use it as an adejective. "A heavy coat can warm you before you catch cold" is an example of warm as a verb (and a "before" thrown in for good measure).