Questions & Answers about Voy a donar todo lo que sobre.
Why is sobre in the subjunctive here instead of the indicative (sobra)?
Because the speaker doesn't know exactly what or how much will be left over. When todo lo que (everything that) refers to something unknown, future, or hypothetical, we use the subjunctive. If they were pointing at a specific, known pile of leftovers right in front of them, they would use the indicative (todo lo que sobra).
Wait, isn't sobre the word for 'envelope' or 'about'?
It can be! But here it is a verb. It comes from sobrar, which means 'to be left over' or 'to be extra.' Because it is an -ar verb conjugated in the present subjunctive, it takes the -e ending for the third-person singular (matching the neuter concept of 'everything').
Why do we need lo here? Can I just say todo que sobre?
No, you always need lo here. In Spanish, you cannot put todo directly next to que to mean 'everything that.' You need the neuter article lo in the middle to represent the unknown or unnamed group of things you are talking about.
Since the leftovers will happen in the future, could I use the future tense and say todo lo que sobrará?