進退兩難 Caught Between Doubt and Belief
那太子聞言，心中慘慼，暗自傷愁道：「若不信此言語，他卻有三分兒真實；若信了，怎奈殿上見是我父王？」這才是進退兩難心問口，三思忍耐口問心。行者見他疑惑不定，又上前道：「殿下不必心疑，請殿下駕回本國，問你國母娘娘一聲，看他夫妻恩愛之情，比三年前如何。只此一問，便知真假矣。」那太子闻言，心中惨戚，暗自伤愁道：「若不信此言语，他却有三分儿真实；若信了，怎奈殿上见是我父王？」这才是进退两难心问口，三思忍耐口问心。行者见他疑惑不定，又上前道：「殿下不必心疑，请殿下驾回本国，问你国母娘娘一声，看他夫妻恩爱之情，比三年前如何。只此一问，便知真假矣。」
逐句解 Line by line
That prince, hearing this, was stricken to the heart, and grieved within himself, saying, “If I believe not these words — yet there are three parts of truth in them;
The boy's honesty is what makes him sympathetic: he does not pretend the story is nonsense, he weighs it. 三分兒 is three parts in ten — he puts the odds at thirty per cent and cannot dismiss them. 慘慼 is stricken with grief; 分 is fēn and 兒 the light er; 心中 takes the locative zhōng; 那太子 takes the pointing nà and 子 as zǐ.
and if I do believe — how then, when he I see upon the throne is my royal father?”
And the other side of it, which is unanswerable: the man on the throne is my father. 怎奈 is "but what is to be done."
This indeed was the strait of advance-and-retreat: the heart questioning the mouth, and thrice pondering in patience, the mouth questioning the heart.
The narrator marks the deadlock with a balanced pair — 進退兩難心問口 against 三思忍耐口問心, the heart questioning the mouth and the mouth questioning the heart, with the phrase reversing itself across the two halves. 進退兩難 — hard to advance and hard to retreat — is still the ordinary phrase for being stuck between two bad options. This is the third and last of the chapter's balanced flourishes, after the king's tears and the pilgrims' sleepless night.
Pilgrim, seeing him wavering in doubt, came forward again and said, “Your Highness need not doubt in your heart. Let Your Highness drive back to your kingdom and ask the queen your mother a word — see how the love between husband and wife compares with three years ago.
Wukong stops arguing and proposes an experiment, which is the cleverest thing anyone does in this chapter. Go home and ask your mother one question. 國母娘娘 is the queen-mother; 疑惑不定 is wavering in doubt; 夫妻恩愛之情 is the affection between husband and wife — the one thing a perfect copy of a man cannot fake to the person who knew him best.
By this one question alone, the true and false will be known.”
Five characters of complete confidence: 便知真假矣. The chapter's whole question — true and false — settled by one domestic enquiry. 只此一問 is "by this single question alone."
The prince's anguish is real and well drawn. He cannot dismiss the story — he puts it at "three parts truth" — and he cannot accept it either, because the man on the throne is visibly his father. The narrator marks the deadlock with a balanced pair that turns back on itself, heart questioning mouth and mouth questioning heart: the state of a son being asked to doubt his own father.
Wukong's answer is not another argument. It is a test, and it is aimed with real psychological precision: go and ask your mother whether your father's love is what it was three years ago. An impersonation that fooled four hundred officials and a whole court cannot survive that one question, because the person best placed to notice has been noticing for three years without knowing what she noticed.