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What is the difference between 'spend' and 'spent'?
Can 'spend' and 'spent' be used interchangeably?
The tense in your sentence is Present Simple. The evening is spent with friends not because of some other tense used in the phrase, but because it is in the passive voice. The passive voice requires the appropriate form of the verb 'to be' + past participle, and spent is the past participle of spend.
The verb "spend", meaning to use time, is a transitive verb i.e. it needs an object to make sense. You can say we spend an evening, not an evening spend/spends. However, you can say an evening is spent (by us) that's in the passive.
So you can say an evening that is spent with friends or an evening spent with friends. The relative clause "that is spent" has been reduced to "spent".
Further, the sentence will also be grammatical if you say "..... an evening (that) we spend with friends".