Command Form (Imperative)

Contract Verbs

Verbs with stem ending in -α

ὁ παῖς βοᾷ, οὐ καθίζει οὐδε σιγᾷ. ὁ οὖν διδάσκαλος λέγει·
The little boy is shouting, he won't sit down nor will he shut up.
So the teacher says:
μη βόᾱ, ἀλλὰ  κάθιζε  καὶ  σίγᾱ
Don't shout (stop shouting) but sit down and shut up.

Why not  ᾱι ?
Because the iota in long vowel diphthongs is not really pronounced any more but sortr of drowned by the long α, the η or the ω.
Therefore, whenever you see a long vowel followed by a iota you know you have to pronounce the 2 vowels separately because they belong to 2 separate syllables.
And whenever you see an alpha with a subscript iota you know that the alpha is long, no need for a macron. Of course, if stressed, it will need a circumflex, a stressed long-vowel diphthong is always circumflexed.

Next, where do we place which accent?
In conjugated verbs the accent is recessive (i.e. moves as far away from the end of the verb as possible). Because the present imperative ending is -ε(sg) -ετε (pl) the accent falls on the antepenult:
ἐρώτα-ε
ἐρωτά-ετε
But if, like in the present indicative, the ending is long, then accent falls on the penult: ἐρωτά-ει
Now comes the interesting bit:
When 2 syllables are merged in a contraction they look like 1 but still count as 2. Which means that, if the accent was on one of them in the uncontracted form it will still be there in the contracted form but acute will change to circumflex.
ά + ει   becomes ᾷ   ἐρωτά-ει     becomes  ἐρωτᾷ  he/she asks
ά + ε    becomes ᾶ   ἐρωτά-ετε   becomes ἐρωτᾶτε  (pl) you ask or: ask!
But if the stress/pitch was not on either of the 2 contracting vowels, it will stay acute
α ε     becomes        ἐρώτα-ε      becomes  ἐρώτᾱ  Ask! (sg.)

Get used hearing the difference:
γελᾷ          feels like he laughs or she is laughing
γέλα          sounds like an order given, i.e. laugh!, short and sharp
μέγα βοᾷ      sounds like SoandSo really does shout rather loudly,
so you say to him/her μὴ  βόα   please stop shouting

You can only "hear" the difference in meaning if you practice aloud, putting the stress where it belongs. Otherwise it just becomes one of those meaningless, boring little grammar exercises of theoretical accentuation. And misses the point totally: the spoken stress (or pitch) makes all the difference. The accent is written to show us how to pronounce the word. Therefore, when you write, to show others which part of the word to stress so as to convey your meaning, mark it: use diacritical marks to make clear both pronunciation and meaning.
The plural you-forms are of course identical in imperative and indicative, so context will have to tell which is which.