1. In the literal sense, to bite back one's tongue would mean biting the tongue to pull it back in one's mouth. As a figure of speech in context, it is a more effective way to say that the speaker should have refrained from asking for more candy, as it is, for some reason, inappropriate. Another sentence with the idiom: I should have bitten back my tongue instead of telling him to go away.
2. In the literal sense, to fall on deaf ears would mean not to be able to listen. As a figure of speech in context, it is an effective way to say that something (a request, an appeal, a wish) is being ignored or disregarded, or the person being spoken to just doesn't want to listen. Another sentence: I said many things in her defense, but all of them fell on deaf ears.
3. In the literal sense, to call it a day would mean that the day is over. As a figure of speech in context, it is a colloquial way to say that the work is over, and one can go home. There is also an equivalent idiom: to call it a night. Another sentence: I've been working for 10 hours now, so I think it's time to call it a day.
4. In the literal sense, a bed of roses would mean a bed covered in rose petals. As a figure of speech in context, it is an effective and picturesque way to say that something is very easy, pleasant, enjoyable, comfortable, etc. Another sentence: They wanted to beat me in solving math problems, but they didn't know it would be a bed of roses for me.