conjugation.subjunctive.present-irregular
Six truly irregular subjunctives: sea, esté, vaya, haya, sepa, dé
Only six verbs escape the yo-form recipe: ser → sea, estar → esté, ir → vaya, haber → haya, saber → sepa, dar → dé.
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Explanation
Because their yo forms don't end in -o (soy, estoy, voy, he, sé, doy), six verbs need their own subjunctive stems: sea, esté, vaya, haya, sepa, dé, then regular endings: seamos, vayan, sepáis.
Estar carries written accents on the stressed forms (esté, estés, estén) and dé takes one to distinguish it from the preposition de.
Everything else (including tener, venir, decir, conocer, the entire -go and -zco families) is covered by the yo-form recipe and is not, strictly speaking, irregular in the subjunctive.
Examples
Espero que sea fácil.
I hope it's easy.
I hope it's easy.
Region: global
No creo que vayan a venir.
I don't think they're going to come.
I don't think they're going to come.
Region: global
Cuando estés listo, avísame.
When you're ready, let me know.
When you're ready, let me know.
Region: global