Whenever I want to prevent someone or something to enter a specific area, can I say "[someone/thing]-proof"?

Why is "be prepared" used as a passive? Is it different from just "prepare"?

"Be prepared" refers to being in a continuous state of preparedness, while "prepare" refers to a one-time action. Ex: Be prepared for a fire in your home by owning a fire extinguisher. Ex: Prepare for the storm that is coming tomorrow.

What does "in years" mean? And how is it different from "for years"?

"In years" generally means the same thing as "for years". The main difference is that "in years" emphasizes the point at which the action last happened, while "for years" emphasizes the time since the action last happened. However, both phrases are very idiomatic and are often used interchangeably, depending on the person or the dialect.

I don't get how "goes" is used here. what does that mean in this context?

Here, "goes" means to proceed in a specific way or with a specific outcome. Ex: How did your week go? Ex: The story goes that the mailman ran away from town and never returned. Ex: We'll see how the night goes. Then we'll decide if we still wanna watch a movie after having dinner.

Can the word "start" be used alone without mentioning what to start?

Yes, in this context, the action referred to by "start" here is implied or already known by the listener, so it's not necessary to say it. In a lot of cases, "start" is used on its own because it indicates that one can begin the action they are meant to be doing. For example, when students are waiting to begin an exam, you can say "start" without telling them the action, because they are already aware. Ex: Ready, set, start! Ex: You can start when the alarm goes off.

What's "likely" mean?

"Likely" is similar in meaning to "probably" and is used to express that something might as well happen or be true. Ex: It's likely to rain all week. Ex: The conservative candidate is likely to win the election.