Check it please

I am writing to complain about your restaurant "Niva", which I visited last Saturday during lunchtime. I was not (Since we are writing a formal letter, we do not use contractions such as "wasn't".) pleased with the service, the prices, or and(When we use "or", we choose one out of a few choices. In your case, you were not pleased with all three aspects -- service, pricing and food quality.) the quality of food.

Firstly, the waiter looked untidy. He was worn wore a pair of torn jeans and a dirty T-shirt. Moreover, he was very unfriendly. We had ordered the fish but it was cold and too salty. Secondly, I had ordered boiled potatoes, but the waiter served us brought chips. Every aspect of the meal and your restaurant was disappointing.

I suggested that your staff will wear uniforms and be friendlier. (You are currently providing your suggestion. It would be in present tense.)

I hope that you will give this matter serious consideration and that such poor service it won't will not be repeated. I hope that your service can be improved will be better, than now or you could lose  (when we use a compound verb, the second verb after the modal verb should be in present tense. Therefore, we write "could lose (present tense), and not "could lost (past tense).) your customers. costumers.

Yours faithfully,

Marya.

Check it please

I am writing to complain about your restaurant, "Niva," which I visited last Saturday during lunchtime. I wasn't pleased with the service, the prices, or the quality of the food.

Firstly First of all, the waiter looked untidy; he was wore wearing torn ripped jeans and a dirty T-shirt. Moreover, He was also very unfriendly. Secondly, I ordered boiled potatoes, but the waiter brought me chips. we had ordered The fish but it I was served was cold and too salty. Secondly, I had ordered boiled potatoes, but the waiter brought chips. In conclusion, every aspect of the meal and of your restaurant was disappointing.

I would suggested that your staff will wear uniforms and be friendlier. [This sentence doesn't fit very well. I would probably omit it.]

I hope that you will give this these matters serious consideration and that it they won't be repeated. I hope that your service will be better, than now improve, or you could lost lose your costumers customers. [OR you could say: If your service does not improve, you are sure to lose customers.]

Yours faithfully, Best wishes,

Mayra

Since you mentioned service, prices, and quality of food in the first paragraph, I think it would help the consistency and flow of your letter if you included each of those aspects in the second paragraph. You mentioned the service and food quality, but not the prices. The second paragraph is also a little strange since the first and second problem are clearly signaled with "first of all" and "secondly," but there's not a clearly marked conclusion. You could talk about the service, the prices, and the food quality, and then sum everything up with your last sentence.

These are just some ideas  I hope they're helpful to you! 

Check it, please

Marya.

Well done Marya! Good writing! 

I see you are a bit confused with the past perfect tense - 

When do we need to use the past perfect tense?

We use the past perfect tense when we want to contrast the consequences of an earlier event to a later one.

For example, 'We had ordered the fish but the waiter gave us chicken instead.' 'I had ordered boiled potatoes but the waiter brought me chips instead.' In these two examples, the word 'instead' is important to show the contrast. 

Other words used with the past perfect tense are 'already', 'yet', 'just' and 'even'. 'I had already eaten dinner when he called to invite me out for a meal.'