Questions & Answers about Vi måste bestämma var vi ska äta idag.
Why is the word order var vi ska äta instead of var ska vi äta?
This sentence contains an indirect question. A direct question flips the subject and verb: Var ska vi äta? (Where will we eat?). But when the question is embedded inside a larger sentence, it acts as a subordinate clause. In Swedish subordinate clauses, the subject comes before the verb, just like in English: ... var vi ska äta ... (... where we will eat ...).
Could I use vart instead of var in this sentence?
No, you need var here because the action of eating (äta) happens at a specific location. Vart means 'where to' and is used for movement toward a destination. If you were deciding where to go, you would say ... vart vi ska åka, but since you are deciding where to eat, you use var.
Why don't the verbs bestämma and äta have present tense endings like '-r'?
Both of these verbs come directly after helper verbs, known as modal verbs. Måste (must) and ska (will) are the active verbs that show the tense. Any verb that immediately follows them must be in its dictionary (infinitive) form. So you get måste bestämma (must decide) and ska äta (will eat).