Ideally we aim to be polite in English even when we are complaining. This is especially true because unlike other languages we do not have a formal version of "YOU", 2nd person singular/plural.
Using "I think..." means you are presenting just an opinion not a fact. Allowing room for both you and them to be wrong.
- Oh, I think you gave me the wrong change.
- I am sorry, I think I ordered the pasta without the tomato sauce.
Using phrases like "I am pretty sure..." allow you to express facts without being too direct.
- I am pretty sure we booked a double room with a balcony.
- If I remember correctly we were told at the time of booking that...
- I believe I gave you a 20 pound note, not a 10 pound note.
We often try to introduce the complaint with an apology or an 'excuse me' or both!
Apologising before a complaint may seem illogical, but it is considered standard and not 'weak'.
- I'm sorry, we ordered the meat well done.
- Excuse me, we have been waiting for half an hour now.
- Hello, excuse me, I am afraid the car you rented us has a problem with the lights.
We often use 'a problem' to introduce the complaint.
- Sorry? We have a problem with our order.
- Excuse me, there is a problem with our room.
- I am afraid there is a problem with this...
- This allows the staff to give your their attention and then you can explain more calmly the problem in detail.
When replying to a complaint it is customary to automatically apologise, even if the customer is not correct.
- I'm terribly sorry, I will speak to the kitchen staff immediately.
- I apologise, I will send someone to your room as soon as possible.
- I am sorry for your inconvenience, I will see what I can do about it.
- I am awfully sorry, however as it says in our terms and conditions we cannot/do not...
- That is a shame, I apologise. Unfortunately, however, we are unable to change it. I am very sorry, it is part of our rules.
Both sides of the discussion may present ideas with modals to not seem too direct and also to add a touch of formality.
- I am sorry, I think you may have misheard our order.
- I apologise for your delay, however you could have informed us you were coming earlier.
- Excuse me, I think you might have put us in the wrong room.