What it means
いつ means “when” and asks about the time at which something happens. It usually appears before a verb or another predicate and can be used in both familiar and polite questions. It asks about a general time or date, rather than a specific clock time.
Usage note
いつ is broader than 何時, which specifically asks “what time?” Also, に is often omitted after いつ when it appears directly before a verb, as in いつ帰る？.
How to use it
いつVerb (ます-stem)ますか
田中さんはいつ帰りますか？
When will Tanaka return?
いつVerb (dictionary form)
田中さんはいつ帰る？
When will Tanaka return?
Example sentences
図書館はいつ開きますか。
When does the library open?
学校の夏休みはいつ始まりますか。
When does the school's summer vacation start?
お母さんはいつ買い物に行きますか。
When does your mother go shopping?
次の電車はいつ来ますか。
When does the next train come?