Breakdown of Гречка осталась, и вечером я разогрела её и добавила овощи.
Questions & Answers about Гречка осталась, и вечером я разогрела её и добавила овощи.
Гречка is a feminine noun (ends in -а), so past-tense verbs agree with it or with a feminine speaker:
- гречка осталась = the buckwheat remained/was left over (verb agrees with гречка)
- я разогрела / я добавила = I reheated / added (verbs agree with the speaker; feminine form implies the speaker is female).
If the speaker were male: я разогрел её и добавил овощи.
In everyday Russian, (что-то) осталось commonly means “some amount was left over / remained (after eating, after an event).”
So Гречка осталась usually implies leftover cooked buckwheat (or prepared buckwheat dish), not that it “stayed” somewhere by choice.
Because this is a compound sentence with two independent clauses:
1) Гречка осталась
2) (и) вечером я разогрела её и добавила овощи
Russian usually separates such clauses with a comma, even if the second begins with и.