You are so right. I'm sorry.
Okay, but in my case the word strong
is an adverb
which describes play
So I'm still not sure, but I really thing We must play more strongly
should be the right form.
This article is probably more appropriate, then: Adjectives Versus Adverbs: Three Degrees of Separation — Infoplease.com
Since 'strongly' is an adverb describing play, the proper form would be 'play more strongly.'
However, I agree with other/previous posts, this doesn't sound natural to me.
I think I'd choose one of these:
"We need to play better."Of these, 'more aggressively' probably comes closest to the meaning of your orignial post.
"We need to play more aggressively."
"We need to play with more speed and agility."
Of course, if it were my son-in-law, he'd probably say, "C'mon guys, let's get these suckers - let's pound them ... are you with me???"