ほど～ない・くらい ②: the difference
ほど～ないN3
Meaning
Is not as…as, Isn't... like..., Not to the extent of...
Past papers
Tested 11 times in past papers
Example
君の母ほど優しい人はいない。
There isn't anyone as nice as your mom.
くらい ②N3
Meaning
To the extent that, So…that, About the only thing that...
Past papers
Tested 3 times in past papers
Example
寝ることしか出来ないくらい、疲れている。
I am so tired that I can't do anything but sleep.
How to tell them apart
「AはBほど高くない」 means B is taller; it equals 「BのほうがAより高い」. It says A's degree is limited, which is gentler than calling A short outright.
In use (1) you can't use 「くらい」: 「×去年の夏くらい暑くない」. In use (2) 「くらい」 is fine: 「健康くらい大切なものはない」.