This was a class I gave recently. The core purpose of this class was to cover language that gives the student of the English language information and techniques on how to be ASSERTIVE yet polite when speaking or writing emails. In this lesson students learnt how to make their message stronger. Being strong or direct does NOT mean being aggressive or impolite.
Assertive : expressing something strongly and with confidence, so that people take notice. ( Oxford Advanced Learners Dictionary)
Ok so this was the class material I covered :
1. Which of these sentences do you think are assertive, and which are agggressive? What’s the difference?
There are some concerns being raised with the standard of your work.
I won’t be spoken to like that – I’m never going to shop in your stores again!
Please send me a full refund, and remove me from your mailing list.
Take your car out of our car park and don’t park it there again!
Your staff are useless – get rid of the lot them.
As a result, we have no option but to terminate the contract.
The language to use when being forceful, assertive, or direct but polite !
When writing a forceful email, you can use these phrases to get the recipient’s attention:
I am writing to remind you that….
I feel you should be made aware of….
I would like to point out that…..
You can use these phrases to make the consequences of the problem clear:
This has resulted in…
This has led to…
This has caused…
As a consequence/ result of this…
Make clear what the consequences will be if there is no action.
If immediate steps are not taken to rectify the situation, we will be forced to consider legal action.
Unless we receive the parts by Friday, it will be necessary to contact another agent.
Please send us assurances that this will not happen again, otherwise we will have no option but to cancel the contract.
So being assertive, strong and direct in our speech in both spoken and written English does not need to be aggressive or angry. It can and should be both polite and professional !
So give it a try. 
More to follow on this topic. Just remember ” IT’S NOT WHAT YOU SAY , BUT HOW YOU SAY IT !
Good luck with your English language learning
Simon
Ok English