A.T. Robertson Commentary John 20:25

"The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my hand into his side, I will not believe." — John 20:25 (ASV)

We have seen the Lord (εωρακαμεν τον κυριον). The very language in the plural that Mary Magdalene had used (20:18) when no one believed her.

Except I shall see (εαν μη ιδω). Negative condition of third class with εαν and second aorist active subjunctive and so as to βαλω (from βαλλω) "and put."

The print (τον τυπον). The mark or stamp made by the nails, here the original idea. Various terms as in Ac 7:44; 1 Timothy 4:12. Finally our "type" as in Ro 5:14. Clearly the disciples had told Thomas that they had seen the τυπον of the nails in his hands and the spear in his side.

I will not believe (ου μη πιστευσω). Strong refusal with ου μη (doubtful negative) and first aorist active subjunctive (or future indicative).