In John 1:15, why is μαρτυρεῖ present tense while ἦν in οὗτος ἦν ὃν εἶπον is imperfect? The tenses seem backwards.
The present tense on past events is common in the Gospel of John, "Historical (dramatic) present indicative of this characteristic word in John (cf. 1:17f.)."* The confusing tense is the ἦν. κέκραγεν is perfect tense with the results of John's cry still ringing in John's memory. But, the imperfect ἦν is as if John the Baptist were still crying out in the present about a past event.
*Robertson, A. T. (1933). Word Pictures in the New Testament (Jn 1:15). Nashville, TN: Broadman Press.