Learn Russian: Как кот ни царапает дверь, это бесполезно. - No matter how the cat scratches the door, it is useless.

This is the concessive construction как ни + verb, which means 'no matter how' or 'however much'. The particle ни here does not make the sentence negative; instead, it emphasizes the intensity or repetition of the action. The cat is definitely scratching, but the effort does not change the outcome.

Can I put как and ни right next to each other, like Как ни кот царапает?

You can put them next to each other, but ни must always go immediately before the verb it modifies. So you could say Как ни царапает кот or keep the current order Как кот ни царапает. However, Как ни кот царапает sounds unnatural because ни is separated from the verb царапает.

Why use the imperfective verb царапает instead of a perfective equivalent?

The imperfective царапает works best here because it emphasizes a continuous, repeated, or ongoing futile effort. The focus is on the process of struggling with the door, which is a classic use for imperfective verbs in these 'no matter how' constructions.

Could the sentence just end with бесполезно instead of это бесполезно?

Yes, both are grammatically fine. Adding это (this/it) provides a concrete subject that points back to the entire first clause (the act of the cat scratching). Dropping it to leave just the impersonal word бесполезно would also be perfectly natural.