Your version isn't wrong at all. It's correct to say 'A firm has a competitive advantage when it implements a strategy THAT competitors are unable to duplicate or find too costly to try to imitate'. We can write this sentence with or without the word 'that'. Both forms are correct. You can choose - it's up to you whether you use the relative pronoun or omit it. If using 'that' makes it clearer, either for you or for your reader, that's fine - go ahead and use it! And yes, there is a rule. Here it is: The relative pronoun is optional if it refers to the OBJECT of the sentence, in this case 'a strategy' . The relative pronoun must be used if it refers to the SUBJECT of the sentence. For example, take a look at the following sentence, which actually contains two relative clauses using 'that'. In both instances, the relative pronoun has to be used because it refers to 'the companies'. 'The companies' is the subject of the sentence, so you have to use 'that' - the same as you would in Italian. The companies that have a competitive advantage are the ones that implement innovative strategies. I hope that helps.

There is not an actual rule for its use. However, the word "that" is more commonly used orally. When writing academic papers, it is recommended to minimize the use of "that". Example: "For reasons that we cannot explain, there are those who don't know that it's important to eat healthy." "For reasons we cannot explain, there are those who don't know it's important to eat healthy." They mean the same thing, but the second one sounds more professional.

This sentence is known as a relative because it gives more information about something without starting a new sentence. It is common to drop 'that' in these cases.

I've noticed a difference in British English and American English. I bet this sentence is from an American source. Simply, they're both completely correct, but I would write it like you do (I'm English). I cannot think of any particular rule that would help.