Chinese · Pronunciation Guide
nah, dropping hard from high to low. Open vowel, ordinary n, and a fourth tone that must be decisive.
nà
The falling contour is the only thing separating this from 哪 nǎ, the question word "which", which shares its vowel and consonant exactly. Tone alone separates "that" from "which", so the descent is not optional. In colloquial speech 那 also fuses with a following measure word to give nèi, a contraction worth recognising.
Listen: 那
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那是我的座位。
Nà shì wǒ de zuòwèi.
That is my seat.
Listen: 那是我的座位。
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那是我的座位。 puts two falling tones side by side at the start, nà and shì, and then wǒ keeps its dip because the following de is toneless. Zuòwèi ends the sentence with two more falls. So four of the six syllables descend, and if you let them slide into one another the sentence becomes a single long downhill rather than a series of separate drops.
The tone is the whole game here. A level or dipping delivery turns "that" into "which", and English speakers routinely level demonstratives because they feel grammatical rather than lexical. Give it the full drop. The vowel is a smaller issue: the flat a of "cat" is too front and too tight — open the jaw and let it be broad.