May.Mag-an, the stem of maht, might, means to strongen, to be or become strong (Lat. valere), as is shown by cases of its use in Saxon and other Teutonic tongues.

In an old Friesic good wish at the drinking to the health of a bride and bridegroom we find ‘Dat se lang lave en wel mage,’ that they long live and well may (strongen, bene valeant); and in Saxon, ‘Hu maeg he?’ how mays he? (strongens or valet).

Shall.Sceal-an, meant, as a stem, to offmark, distinguish, or to skill in the meaning of 1 Kings v. 6—‘Ic sceal dón,’ I offmark or skill to do; as what I am bent to do.

‘Thou shalt love the Lord thy God.’ Thou markest or clearly seest to love the Lord thy God.

‘Thou shalt not steal.’ Thou markest this. Not to steal.

Must.Mot-an, most-an, is most likely a stem of the word mag-an, to strongen (valere).

The -st would strengthen the meaning of mag (may) as it does in -est of longest. So ‘I must go’ (Ic moste gán) would mean ‘I am overmighted by another’s might to go.’

Time-words are fitted

To Person, as
I am.Thou art.He is.
To Tale, as
I am.
We are.
Thou art.
Ye are.
He is.
They are.
To Time, as
I am (now).I was (heretofore).
I shall be (hereafter).

To Mood, as