DON'T WRITE LIKE YOU TALK

Do not write like you talk: 

When you write the way you speak, it is called informal or conversational writing. While it may be appropriate for texting or informal emailing, it’s generally not the clearest or most effective way to convey your thoughts.

Speaking tends to be more casual than writing, while writing is more formal (even in neutral/less formal text types). 

When we speak, we’re less likely to use correct grammar and often include partial sentences. Speech is generally more spontaneous than writing and therefore you are more likely to stray from the subject under discussion (to be off-topic).

When writing, for example, 

Learning to write is a skill which requires a lot of practice, and using the right conventions for each text type