↑ "The present perfect tense | Learning English Grammar | Collins Education". grammar.collinsdictionary.com. Retrieved 2018-08-26. ↑ A Practical English Grammar, A.J.Thamson and A.V.Martinet 2001, ISBN 0194313476 P 166. templatestyles stripmarker in |title= at position 65 (help) ↑ Comrie, Bernard, Tense, Cambridge Univ.