Learn Polish: Czy znasz angielski? - Do you know English?

Znać belongs to this exact group! To conjugate it for I and you, you drop the -ć from the infinitive and add the new endings. I know is znam, and you know (singular and informal) is znasz.

We just learned umieć (to know how to). Could I use it here instead of znasz?

Not with this exact sentence structure. Znać is used when you know a noun, like a language, a person, or a place (znasz angielski). Umieć is for learned skills. If you want to use umieć, you need to pair it with an action verb or an adverb, like Czy umiesz mówić po angielsku? (Do you know how to speak English?).

I often see po angielsku for English. Why is it just angielski here?

It depends on the verb! When you use action verbs like to speak (mówić), you use po angielsku (which translates roughly to in the English way). But znać (to know) requires a direct object. Here, you are saying you know the English language as a thing, so you use the noun form angielski.

What is the word Czy doing at the start of the sentence?