The following is from the hellenisticgreek website:
“Verbs whose present tense stem ends with a liquid or nasal consonant (λ, ρ, μ, ν) undergo predictable spelling changes. Just as with the σ for the first aorist, the σ for the future is omitted with these verbs. Instead of adding σ, the accent is moved from the verb stem to the ending.”
What takes place here seemed to me to be identical with what takes place in the case of present tense conjugation of ε-contract verbs.
Consider κρίνω. If this were an ε-contract verb (κρινέω), we would expect its present active indicative forms to be κρινῶ, κρινεῖς, κρινεῖ, and so on, which incidentally is precisely what its future active indicative forms correspond to.
So can I think of this as a general rule? In other words, are liquid and nasal futures identical to present indicative forms of ε-contract verbs?
And can you see why they are identical? Good reason for knowing your principal parts, too!
No, unfortunately I cannot see why they are identical.
And here is a related question: is it the case that in ancient Greek a σ is never placed directly after a liquid or nasal consonant? Since I have never seen it, I am wondering if this can be considered a general phonological rule.
Yes, futures with nasal and liquid stems are effectively a kind of contract verb.
To refresh myself of the details, I’ve looked up the matter in Buck’s 1933 Comparative Grammar of Latin and Greek. Sihler’s Comparative Grammar would be more up to date, but I understand Buck still stands on this issue, and in any case I only have him to hand. I did manage to find Buck online, and would link to the book, but I have not yet come off my Textkit forum training wheels, and links are a no-no!
Historically speaking, liquid/nasal stems are understood to have formed the future with -εσω rather than just -σω (e.g. *κρινεσω). (Buck section 389-90) A sigma between vowels dropped out (Buck 164), leaving κρινέω/κρινῶ in our example, where the form is identical with the epsilon contract verbs in the present tense. I presume this is what Barry was hinting at.
As for your related and pertinent question, Buck would agree with you that σ does not come after a nasal. He describes that -μσ- became -νσ- (195), and that in the cluster -νσ- the sigma is dropped, and that its preceding vowel is lengthened. (203) The κρίνω example above is a good example of this, since the Attic Greek aorist ἔκρῑνα is understood to have derived from *ἐκρινσα, with the usual -σα aorist ending. With liquids the picture is a little more complicated, but it is not quite correct to say liquids could not be followed by σ, since forms exist such as ἄλσος, τέλσον, ἔκελσα and ὦρσα.
In response to the OP, I would add that verbs ending in an iota/zeta/omega combination most commonly form the future tense in this way too. eg. vomisdw > vomiew > vomiw.
The future stem of the liquid/nasal verbs, however, although similar, is not always the same as the present stem.