Actually is not so important, for example: in spanish, a lot of people forgot the grammar and they have grammar mistakes too when they speak, write, etc..

"He make fish in dinner out his " This would be a sentence written grammatically wrong ,so if you don't learn grammar at all ,it would be a mess and your sentences wouldn't make sense to the listener. ,but anyways on your way to learn a language you will not master all the grammar at once so you will be making mistakes but the more you advance the less mistakes you ll be making So grammar is an essential part of the language's make up !

Grammar is an important part to learning any language. Without proper grammar people can be easily confused by your word structure as it doesn't follow the natural patterns they listen for. Instead of understanding what you say as you say it, they hear what you say but can't understand until after you're finished (and they think about it), assuming they understand at all.