It's an aspect distinction.
να λέω is the imperfective form (I say in general, repeatedly, habitually)
να πω is the perfective form (I say once)
I think it is similar to the distinction between говорить and сказать.
Here, the idea is probably that I am trying to tell him once, so we use the aorist subjunctive να πω rather than the present subjunctive να λέω.
They are both considered forms of the same verb in Greek (dictionary form: λέω).
λέω=I say, έλεγα=I was saying, είπα=I said, θα λέγω ή θα λέω=I will be saying, θα πώ=I will say, θα έλεγα=I would say, έχω πεί=I have said, είχα πεί=I had said, θα έχω πεί=I will have said, θα είχα πεί=I would have said. For any further information leave a note here, I will be notified by DL. Good luck!
Ok, then we return to your question, learn the essential: PRESENT: λέω, λες, λέει, λέμε, λέτε, λένε PRESENT PERFECT:έχω πει, έχεις πει, έχει πει, έχουμε πει, έχετε πει, έχουν πει FUTURE: θα πω, θα πεις, θα πει, θα πούμε, θα πείτε, θα πουν. Now ΠΙΘΑΝΟ means possible if you wanted to say «I cannot» you would say «δεν μπορώ». For me too english is difficult, every language it is, it contains the wisdom of centuries but it is also a wonderful journey!
Yes, nearly all verbs have two separate (but usually related) stems for the perfective/imperfective distinction.
The aorist stem often includes a -σ-, which may fuse with a preceding consonant into -ψ- or -ξ-.
Λέω is one of the more irregular verbs in that the two stems bear little resemblance to each other. Others are e.g. θα βλέπω, θα δω - θα πηγαίνω, θα πάω - θα τρώω, θα φάω - θα πίνω, θα πιώ (imperfective/continuous/habitual first, then perfective/single-event).
But e.g. θα γράφω, θα γράψω - θα τρέχω, θα τρέξω - θα διαβάζω, θα διαβάσω are more regular.
A few verbs do not have separate stems and/or only exist in the imperfective form, e.g. θα είμαι - θα θέλω.