greek syntax principle 12

The last vowel of the stem of many verbs will lengthen when it comes into contact with the tense formative (BBG 19.12). Alpha and epsilon both become eta; omicron becomes omega.
αγαπαω – present tensThe alpha is the last letter of the stem. When the tense formative is brought on (σ), it lengthens to an η.e
αγαπησω – future tense.
Note the tense formative caused the α to become η.