Questions & Answers about Igår glömde han exakt vilket datum det var.
Why does the sentence end with det var instead of var det?
Because vilket datum det var is an indirect question acting as a subordinate clause. In a direct question, the verb comes before the subject (Vilket datum var det?). But in a subordinate clause, the subject always comes first (det var).
Why is it glömde han (forgot he) instead of han glömde (he forgot)?
Because the sentence starts with the time word Igår (yesterday). Swedish uses the V2 rule, meaning the verb (glömde) must be the second block of information in the main clause, so the subject (han) has to move after it.
Why is it vilket datum and not vilken datum?
Datum is an ett word (ett datum), so the question word 'which' or 'what' must match it by taking the -t form: vilket.
The English translation says 'what date', so why does the Swedish sentence use vilket (which) instead of vad (what)?
In Swedish, when you are asking 'what [noun]' or 'which [noun]', you use vilken, vilket, or vilka. You generally only use vad by itself to mean 'what' (like Vad var det? - What was that?).