Interesting question: Why do I struggle to speak English even though I am quite good at grammar?  

The honest answer to that question is likely because you are quite good at grammar.

You've probably focused on it too much,

spent too much time studying it.

And therefore, are overly reliant on it, and you have been trying to train yourself to speak English by using those grammar rules.

But the problem is that does not play nicely with what's supposed to happen in here to make this happen smoothly.

There was a time when we thought native speakers of any language, not just English,

were processing that language using grammar rules and individual words, computing them in the mind to create sentences step-by-step as they went.

But that never really made that much sense.

Then in 1983, the researchers Pawley and Syder published their seminal research work.

Two theories, or two puzzles, for linguistic theory: Nativelike fluency and Nativelike selection.

And in that paper, they argued that native speakers shouldn't be able to speak so fluently

because if we were using grammar rules and individual words, computing them, that would happen in working memory.

The conscious part of the mind, which is extremely limited - It's really not that powerful.

Therefore, we shouldn't be able to speak so rapidly, so fluently without screwing everything up every five seconds.

Not only that but grammar doesn't explain how we sound so natural.

Why do we say, 'Could you help me with this'? instead of, 'Would you aid me in this task'?

Both are equally grammatical, one sounds very weird, the other sounds very natural.

Why do we say 'good morning' instead of 'pleasant first half of the day'?

Same situation: One sounds extremely strange, the other sounds very natural.

Well, the answer to this question and the fluency puzzle is that

we don't default to computing sentences using grammar rules and individual words at all.

What we actually do is make use of a different part of the mind,

not working memory or conscious memory but long-term memory, which is immensely powerful.

We store large blocks of language in long-term memory that we can just pull out and line up as we speak as wholes

without having to compute or analyze or do any kind of difficult mental gymnastics as we go.

These chunks of English are how we speak fluently and how we speak naturally.

And if you, the second language speaker want to learn to speak in the same kind of way in English, as you already do in your first language,

you need to switch your focus from grammar and vocabulary, what you were taught in school.

And instead, start seeing and learning and using English as a system of high-frequency, highly natural chunks instead. 

And once you get good at this, fluency and naturalness in the language will follow.

I'll see you on the other side. And this is me, Dr. Julian Northbrook signing out from another video for another day.

See you later, guys. Bye-bye.