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

References

156. Future Active and Middle of Stems Ending in a Consonant

When the stem of a verb ends in a consonant, the addition of the tense suffix σ brings two consonants together. The following results then occur:

(1) π, β, φ (called labial mutes because they are pronounced by means of the lips) form with the following σ the double consonant ψ.

Examples: The future of πέμπω is πέμψω, and of γράφω, γράψω.

(2) κ, γ, χ (called palatal mutes because they are pronounced by means of the palate) form with the following σ the double consonant ξ (ks).

Examples: The future of ἄγω is ἄξω, and of ἄρχω, ἄρξω.

(3) τ, δ, θ (called lingual mutes because they are formed by means of the tongue) drop out before the σ.

Example: The future of πείθω is πείσω.