We use "let alone", "much less" and "still less" when we want to emphasize that because the first thing we mention is not true or possible, the second thing is even less likely.
They translate into Spanish as "y mucho menos".
Example:
I hardly have time to think these days, let alone go to the gym.
Apenás tengo tiempo para pensar estos días, y mucho menos para ir al gimnasio.
In the picture below, you will find some more examples that will help you understand how to use these phrases.
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