New Testament Greek for Beginners

(Based on the book with the same title by Gresham Machen, The MacMillan Company, 1923)

References

151. The present and imperfect tenses, in all three voices, are formed on the present stem, to which the personal endings, being joined to the stem by the variable vowel ο/ε, are added.

But the future active and middle are formed on the future stem, which is formed by adding the tense suffix σ to the stem of the verb. Thus, while λυ- is the stem of the verb (which in the case of λύω is also the present stem), λυσ- is the future stem.