You should have started by now!

Why hasn't he started yet?

The task was introduced by a keen, vibrant teacher.  Visual prompts were used and real-life examples given to engage the class.  The children responded to their learning with enthusiasm.
So, why has Jimmy still got a blank page in his book 5 minutes later?
Why does this child, who has clear ideas when sitting on the carpet, not manage to get started on tasks of which the teacher is certain he is capable?

I am using Jimmy as an example (sorry to anyone named Jimmy reading this!) simply because most of the children I have come across with executive function problems are boys, by no means all, but certainly the majority.

Executive Functions are a suite of basic tasks which we deal with every day.  The precursors, if you will, to success in everyday life.  The building blocks upon which we build our daily lives.  Neurological diversities, in the form of Dyslexia, Dyspraxia, Dysgraphia, Dyscalculia and Autistic Spectrum Conditions often encompass an executive function deficit in one or more areas, as do, I would argue, most humans.

Executive functions are not, in this context, anything to do with corporate entertainment or company cars!

Consider a simple classroom writing task from Jimmy's perspective:  Could you follow a task if you didn't first locate the page, the line on the page, find the pencil and hold it correctly, know how much pressure to exert on the pencil in order to create a mark and know how to prevent the pencil destroying the paper from too much pressure? Do you understand how to produce shapes from the pencil, in which direction you need to move the pencil, how to form a diagonal line and how to reverse over the same line to change direction? 

Phew! All this before we even need get to understanding the alphabetic code! 
Oh, and I didn't even mention the decision to stand, sit, lean on a hard surface to support the page, or understand the purpose of a table!

Consider the working memory required to hold the instructions given by the teacher in your mind long enough for you to be able to manipulate them and carry out a task. 
Repeating daily basics like date and learning objective may help reduce the number of instructions a child needs to remember but this is why we need visual prompts.

One of the children I work with showed me how she copes with getting started:

'DUMTUMS'
Date
Underline
Miss a line
Title
Underline
Miss a line
Start

You next need to plan your work, to know what you are going to write.  I am not referring to the 'story plan' format used by many teachers to help children think about their writing.  Long before that an inner conversation takes place in your brain such; "Okay, I want to write 'cat', so what do I do first? Pick up the pencil, ok, ready, where am I going to write? Top line? Which is the top? How far away from the very top do I need to write? Where does the page start? Where does it end? Shall I stop here, or here? OK, ready. 
What was I going to write again?"

Are you starting to feel Jimmy's pain?

Now he needs to get down to the 'secretarial' skills.
He has recorded his sentence on a handy recording device such as a Talking Postcard to support his working memory, so what to write is sorted.
Press play 'The cat sat on the mat'
Right, 'cat'.  First sound, /c/ which grapheme (picture) matches that sound c or k?
How big is a c? Which direction does the pencil move? Does it touch the top line?  Does it rest on the line or drop below it?  Much easier to make it float in the middle. 
Next letter, direction, where? Does it touch the other letter? Bigger? Next to the c, where is 'next to'....

Finally, at the end of playtime, after all his friends have finished, Jimmy takes his writing to show the teacher.

"Good, now go back and check for capital letters and full stops"

Jimmy spends the rest of the day with his head on the desk.