在英语中，"防晒比较弱" 可以用 "not very effective at blocking UV rays" 来表达。
"Effective" 是形容词，意思是有效的、起作用的。"UV rays" 是紫外线的意思。"Block" 是动词，意思是阻挡、隔绝。
例句：
This sunscreen is not very effective at blocking UV rays. It's better to use a stronger one. (这款防晒霜不是很有效，最好使用更强的防晒霜。)
I appped the sunscreen, but I still got a sunburn. It must not have been very effective at blocking UV rays. (我涂了防晒霜，但我还是晒伤了。那肯定不是很有效。)
"Effective" 是形容词，意思是有效的、起作用的。"UV rays" 是紫外线的意思。"Block" 是动词，意思是阻挡、隔绝。
例句：
This sunscreen is not very effective at blocking UV rays. It's better to use a stronger one. (这款防晒霜不是很有效，最好使用更强的防晒霜。)
I appped the sunscreen, but I still got a sunburn. It must not have been very effective at blocking UV rays. (我涂了防晒霜，但我还是晒伤了。那肯定不是很有效。)