when to use relative pronouns 'that' &'when' in a statement?

Time..........(that / which / when) is lost is never found again.

explain with reason.

We use “that” as a relative pronoun with restrictive clauses. Restrictive clauses limits the subject or makes the subject more specific. We do not use a comma before “that”.

We use “when” as a relative pronoun when we mean to say “since what time?” For example: “My mother fears water, since when my grandfather drowned in the river in front of her and she could not help him.”

“Which” is used as a relative pronoun with non-restrictive clauses. A non-restrictive clauses talks about the subject generally and not specifically. We use a comma before "which".

If we take the sentence provided by you as an example, here “that” would be the most appropriate answer. Here “Time has been specified as something that is lost”. "Which" is inappropriate because there is no comma in the sentence. “When” is totally incorrect as it makes no sense.

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Comparison of the use of That, Which and Who
The use of the relative pronouns thatwhich and who is summarized in the following table.

Relative PronounType of ClauseType of Antecedent
  that  defining clause only  persons or things
  which  defining or non-defining  things only
  who/whom/whose  defining or non-defining  persons only


From the preceding table it can be inferred that in the case of defining relative clauses, that may be used to replace whowhom or which. For instance, the following sentences:
      The boy whom we saw is her brother.
      The hat which you are wearing is rather large.
could be rewritten:
      The boy that we saw is her brother.
      The hat that you are wearing is rather large.

Like the relative pronoun thatwhom and which can generally be omitted when they act as the object of the verb in a relative clause. Thus, the preceding sentences could also be rewritten:
      The boy we saw is her brother.
      The hat you are wearing is rather large.

It should be noted that when whom or which is the object of a preposition, the preposition immediately precedes the relative pronoun.
e.g. The boy to whom we sent the message was excited.
      The room to which you will be conducted has beautiful furniture.
In these examples, whom and which are immediately preceded by the preposition to.

However, when the relative pronoun that is the object of a preposition, the preposition is normally placed at the end of the relative clause. For instance, if that is used, the second example must be rewritten as follows:
      The room that you will be conducted to has beautiful furniture.

Therfore answer is that for the question you have asked.

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