更不用说 (let alone / not to mention)

Not even the basics

• Your nephew is two years old and is learning to talk.
• he / him他tāeven连liánold variant of 话话huà(even)都dōuvariant of 说说shuōnot不búclear清楚qīng chǔ，more更gèngneedless to say不用说bú yòng shuōto write characters写字xiě zì(completed / change)了le。 — "He can't even speak clearly, let alone write."

B …，more更gèngneedless to say不用说bú yòng shuō A ((completed / change)了le)

• more更gèngneedless to say不用说bú yòng shuō A = "let alone A, not to mention A": the easier case B comes first; A is even more obviously true or false.
• (completed / change)了le often ends the clause: more更gèngneedless to say不用说bú yòng shuō…(completed / change)了le.

I我wǒeven连liána bike自行车zì xíng chē(even)都dōunot不búcan会huìride骑qí，let alone更不用说gèng bú yòng shuōdrive开车kāi chē了le。

Positive: not to mention

• here这里zhè lǐ(modifier / structural particle)的desummers夏天xià tiānvery很hěnhot热rè，more更gèngneedless to say不用说bú yòng shuōAugust八月bā yuè(completed / change)了le。 — "Summers here are hot, not to mention August."
• he / him他tācan会huìvariant of 说说shuōEnglish (language)英语yīng yǔ、French (language)法语fǎ yǔ，more更gèngneedless to say不用说bú yòng shuōChinese language中文zhōng wén(completed / change)了le。 — "He speaks English and French, not to mention Chinese."

here这里zhè lǐ的desummers夏天xià tiānvery很hěnhot热rè，not to mention更不用说gèng bú yòng shuōAugust八月bā yuè了le。

Easier case first

more更gèngneedless to say不用说bú yòng shuō versus let alone何况hé kuàng

• let alone何况hé kuàng is similar and more literary: adults大人dà rén(even)都dōuto make做zuònot to arrive不到bú dào，let alone何况hé kuàngchildren孩子hái zi(and …? / softener)呢ne？ — "Even adults can't do it, let alone children."
• let alone何况hé kuàng often ends with (and …? / softener)呢ne as a rhetorical question.

adults大人dà réneven都dōucan't do it做不到zuò bu dào，let alone何况hé kuàngchildren孩子hái zi呢ne？

Recap

Read the full reference: HSK 5 grammar — Let alone.