"The city was going about its business without knowing the aliens arrived."

[jutfrank]

would you please, tell me why it's not correct if I followed the rule- past simple is used to talk about an action that interrupts and past continous is used to talk about an action that is interrupted

It's because that isn't what the sentence means. The sentence doesn't describe an action that is interrupted by another action.

The city was going about its business [This describes a past situation]

without knowing the aliens had arrived. [The aliens arrived before the past situation, so you need past perfect]


Perhaps you're misunderstanding the sentence to mean this:

The city was going about its business when the aliens arrived.

In this sentence, the action expressed by the aliens arrived interrupts the ongoing situation of The city was going about its business.

 

[Piscean]

1, The city was going about its business without knowing the aliens had arrived.
2. The city was going about its business when the aliens arrived.

1. The city was going about its business. The aliens were already there.
2. The city was going about its business. The aliens arrived during this activity.

 

[mella@mella]

You don't seem to have taken any notice of my previous corrections. You misspelled "continuous" for the third of fourth time. You used a comma instead of a full stop twice. You missed out an article. You failed to capitalise the first word of a sentence. Please go back and re-read all the previous posts and look very carefully at the corrections. You need to get the basics right before worrying about anything else.

I've started learning English just two months before, please be patient with my mistakes.I'll do my best.

 

[emsr2d2]

I've I started learning English just two months before ago. Please be patient with my mistakes. space needed here I'll do my best.

 

[Rover_KE]

Would somebody please fix the typo in post #3?

Done.