По-английски literally means "the Anglia way" , Англия being the word for England or, informally, for the UK. There is no such word as "английски" . I'll give you another example of using this word-formation pattern. ньюйоркский = of or related to New York (city or state), по-ньюйоркски = in a New York way. The verb говорить can only be modified by an adverb or a noun phrase
могу is pronounced close to muh-goo. The Russian "y" requires protruding your lips much more than the English oo, though. It sounds more like oo in "woo" or ou in wound (injury) than like oo in "good". Moгу may mean "I can", "I may" or "I have an opportunity". Умею is more specific: it means "I can" in the sense of "I know how to", "I am capable of".