The Analytical Subjunctive with the mood auxiliary should is used here; the Past Subjunctive of the verb to be or the Past Indefinite of all other verbs can be also used here.

Тебе давно пора понять, что она тебе не пара.
It's high time you should understand that she is not your match.
It's high time you understood she is not your match.

Ему давно пора быть здесь.
It's high time he should be here.
It's high time he were here.