“The company was established in Silicon Valley in 1983.”
You're writing a report about a company. You're introducing the company and need to say when it was started, so you begin by writing this.
The company was established in Silicon Valley in 1983.
In writing, you should only use "the ___" after you have already introduced something earlier. In this example, you would first use the company's name, and then afterwards you can call it "the company".
It was started in 1983.
In both cases, you use "was ___ed". You use this structure when you don't want to say who did something. In this example, you don't want to say who established the company because that's not important information for your report. If you did want to say who started the company, you would write:
James and Frank Calloway established the company in 1983.