bBa-dDa-bBa-bBa-bBa

Capital letters for titles.

bBa-da-ba-ba-ba...

Capital letters for the beginning of a sentence.

On this day, Aa restaurant that you are lovin' itg opensed its door for the first time in 1940?.

'Opened its door', you're talking about the past.
It would be better to say 'loving', especially if it was formal writing, such as for a letter or an essay.

Can you make a guess?

It would make more sense to just say 'Can you guess?'.

iIt's the American Corporation McDonald's that serves as a fast food company in more than 50 countryies worldwide!

Capital at the beginning of the sentence.
'As a' not 'as'.
Plural of country is countries, and you're talking about more than one here, therefore you say '50 countries'.

(I'm lovin' it) is the predominant McDonald's slogan, and it's started as an ad program in 2003, t. This slogan quickly gained it's popularity in the uUnited sStates, and it's started to be the main slogan of the company and i. It has crossed boundaries to be translated into more than 47 countries where McDonald's operates.

There's a difference between 'it's' and 'its'.
'It's' is 'it is', and 'Its' is what to say when it belongs to something, for example 'Its home'.
Capital for countries. 'United States'.

This slogan is special in its every word, g. Grammatically, the verb (love) is a stative verb that means it's remains as it is in present continuous, and there is no -ing will be added, h. However, lovin' breaks this norm, and with the dropped 'g', makes it just perfect.

But tThat doesn't stop here rthough! Rather, the pronoun (it) refers to something particular, which is McDonald's!

You can't start a sentence with connectives, such as 'but' or 'so.
Commas and capital letters!!

This slogan has made a huge impact and has not only accelerated McDonald's advertising as never before. but maybe advertising as a whole.!

dDespite the coolness of the McDonald's slogan, Hhave you ever eaten at McDonald's?

and was were itWas their food as cool as their slogan?

You can't begin a sentence with 'and'.
You're talking about the food belonging to them, therefore you say 'their food'.
'As cool as' not 'cool as'.

On this day, Awhich restaurant thatwhere you are "lovin' it" opensed its door for the first time in 1940?

Since you were asking a question, you need a question word at the beginning of the sentence, so I put "which". This is a good resource with when to use the different question words:
https://www.englishclub.com/vocabulary/wh-question-words.htm

iIt's the American Ccorporation McDonald's that serve, as fast food company with locations in more than 50 countryies worldwide!

Beginning of the sentence always needs to be capitalized
"corporation" is a noun, it doesn't need to be capitalized unless if it's a proper noun (as in, it's a part of a name)

singular: country (1)
plural: countries (more than one)

The rest of the sentence I changed to make it sound better. When we talk about restaurants having chains in different places, we say for example they have "many locations throughout the country."

("I'm lovin' it)" is theMcDonald's predominant McDonald's slogan, and it's started as ad programslogan, which was first heard in an advertisement in 2003, t. This slogan quickly gained it's popularity in the uUnited sStates and it's started to b became the main slogan of the company and it has crossed boundaries t. The slogan has also been translated into different languages for more than 47 countries where McDonald's operates.

"I'm lovin it'" is a saying, so it should be used inside of quotations

United States is a proper noun, so it should be capitalized

The rest of the sentence I changed to make sound smoother. I recommend that you look up when to use the word "which" in a sentence.

This slogan is special in its every word, g. Grammatically, the verb ("love)" is a stativec verb that, which means it's remains as it is in present continuous, and there is no -ing willto be added, h. However, lovin' breaks this norm and withby the dropping thed 'g', makesing it just perfect.

When you are referring to something, put it in quotations (like "love") instead of parenthesis.
It's called a "static verb", not a "stative verb"
The rest I changed to make it sound smoother/just small grammatical things.

But that doesn't stop there - rather the pronoun ("it)" refers to something particular, which is McDonald's!

Again, use quotations here instead of parenthesis

Also, when you say things like "that doesn't stop there", or "you guessed it", etc. where vocally there would be a pause, you need something to break apart the sentence, whether it's a comma, a dash, 3 dots ...

This slogan has made a huge impact and hasby not only accelerateing McDonald's advertising aslike never before, but maybe advertising as a whole.

I added in "not only" because you aren't just talking about McDonald's advertising, but also other advertising.
Here's another example:
Not only can you lose weight from exercising, but you can also receive other health benefits, such as lower blood pressure.

dDespite the coolness of McDonald's slogan, Hhave you ever eaten at McDonald's?

"eaten" because you are asking if they did something before -> past tense

and was were itWas their food as cool as their slogan?

When you make a comparison, you have to also put an as before the thing you are comparing (in this instance, you are comparing the "coolness")
Here's another example:
Is riding a bike as exciting as riding a motorcycle?