What English do you want to learn?  What do you need to learn?  When can you say "I know English"? 

How many Englishes are there? 

• The Englishes spoken in different parts of the world (see article).
• the English tested in exams (IELTS, TOEFL, national exams, etc.)
• Academic English is formal, written with complex sentence structures.  Medical English and legal English use technical words (jargon).  Many medical and technical words are 'international', nearly the same in many languages so can be understood easily.
• English used (written) in MSN, text messages or online chat by people from different countries: quick shorthand,
• English spoken by others - Two astronomers, one German, one French speaking English: the grammar rules are broken, but both understand because of the common subject.
• The English used by participants on international projects is again different: Participants don't know each other, many have little experience of going abroad, speaking English, or trying to understand other foreigners speaking Italian/Polish/Arabic/Russian/... English, each with different typical mistakes.

Native speakers of English use words differently in spoken and written English according to research. 

Spoken English is difficult for many learners; see research. Students with certain disabilities may have greater difficulties, because they cannot access information (spoken, written) or express information in standard ways.