Irregular Subjunctive: Seja, Esteja, Tenha, Vá, Faça

Some very common verbs have irregular subjunctive stems that have to be learnt by heart:

• ser → seja, estar → esteja, ter → tenha, ir → vá
• haver → haja, saber → saiba, querer → queira, dar → dê

Many others still follow the eu rule, even though their eu form is irregular: faço → faça, digo → diga, posso → possa, vejo → veja, venho → venha, ponho → ponha, trago → traga, ouço → ouça, peço → peça, saio → saia, perco → perca.

The endings are the usual ones: seja, sejas, seja, sejamos, sejam. For ir, the forms are vá, vás, vá, vamos, vão.

Oxalá (let's hope, if only) is always followed by the subjunctive, and so is duvidar que (to doubt that).
Oxalá faça bom tempo amanhã. (Let's hope the weather is good tomorrow.)