1. As an interrogative adverb, it leads to a special interrogative sentence meaning "when; When ".

(1) When will they come back?

When will they come back?

2. As a connecting adverb, it leads a noun clause or infinitive, indicating "when". Clauses use declarative sentence word order, and the tense depends on the actual situation.

I wonder when they will come. I wonder when they will come.

Can you tell me when the bank opens? Can you tell me when the bank opens? (When introducing the object clause)

As an interrogative pronoun, it is often used with prepositions such as since, till and up to express the meaning of "when".

(1) When did they own this house? When did they start to own this house?

When is the library open? How late is the library open?

4. As relative adverbs, guide restrictive attributive clauses and non-restrictive attributive clauses.