that vs which

Relative clauses in the English language are formed principally by means of relative words. The basic relative pronouns are who, which, and that; who also has the derived forms whom and whose. Various grammatical rules and style guides determine which relative pronouns may be suitable in various situations, especially for formal settings. In some cases the relative pronoun may be omitted and merely implied ("This is the man [that] I saw", or "This is the putter he wins with").
English also uses free relative clauses, which have no antecedent and can be formed with the pronouns such as what ("I like what you've done"), and who and whoever.
Modern guides to English say that the relative pronoun should take the case (subject or object) which is appropriate to the relative clause, not the function performed by that clause within an external clause.

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