Addition of ε.—a. To the verb-stem ε is added to make the present stem in δοκέω
b. In many verbs ε is added to the verb-stem to form the tense-stems other than present, second aorist, and second perfect, e.g. μάχομαι (μαχ-)
c. In some verbs ε is added to form one or more tense-stems, as μένω (μεν-)
d. Some verbs have alternative presents with or without ε. Here sometimes one is used in prose, the other in poetry, sometimes both are poetic or both used in prose. Thus, ἕλκω
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].