Be not terrified (mē ptoēthēte). First aorist passive subjunctive with mē from ptoeō an old verb to terrify, from ptoa, terror. In the N.T. only here and Luk 24:37.
First (Prōton). It is so easy to forget this and to insist that the end is “immediately” in spite of Christ’s explicit denial here. See notes on Matthew 24:4-42; note on Mark 13:1-37 for discussion of details for Luke 21:8-36, the great eschatological discourse of Jesus