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Temporal adverb clauses (here, 'when') express future events in the present tense.

Literal

Time [possessive-の] adverb-clause (here [topic-では] 'when') [topic-は], future [possessive-の] matters [object-を] present-tense [with-で] express.

時の副詞節 'temporal adverb clause' — a grammatical concept. The parenthetical ここではwhen specifies the example. A concise grammar rule statement about English tense usage in temporal subordinate clauses.