Japanese · Pronunciation Guide
A single "so" held for two beats — the u in the spelling marks length, nothing more.
sou
One syllable spread across two counts, with the pitch falling as the vowel proceeds. Everything distinctive about this word is duration, so a short version simply reads as a different, incomplete sound to a Japanese listener.
Listen: そう
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そうしないでください。
Sou shinaide kudasai.
Don't do it like that.
Listen: そうしないでください。
Free browser voice. Hear it read by a real native speaker inside the LingoDrop app.
そうしないでください runs the long vowel straight into しないで, where な and い are separate beats rather than the English diphthong "nye." So the sentence asks you to hold one vowel and then to keep two others apart, in the space of a few morae.
"so" in one quick beat is the default, since English hears a familiar word and gives it familiar timing. Double it. Also watch the vowel itself: English "so" glides toward w as the lips close, while Japanese holds one position for the whole two counts. If your lips move, the vowel is wrong as well as short.