Future Tense
εἶμι, I shall go

Totally unexpected forms:
Compare the forms I am (present tense) with I'll go (future tense)

It's starting to rain:
  is outside   Who?   will go inside  
ἔξω εἰμί   I εἴσω εἶμι
εἶ   you (sg) εἶ
ἐστίν     he, she, it    εἶσι 
ἐσμέν   we ἴμεν
ἐστέ  you (pl) ἴτε
εἰσίν they ἴᾱσιν  
 
And the compounds:
  • ἀπέρχομαι - ἄπειμι, I'll go away
  • ἐπανέρχομαι - ἐπάνειμι, I'll go back
  • εἰσέρχομαι - εἴσειμι, I'll go in
  • ἐξέρχομαι - ἔξειμι, I'll go out
  • προσέρχομαι - πρόσειμι, I'll go up to
    καὶ τὰ λοιπά