The Present perfect tense

Why is it wrong to use past simple with the 'in the previous year'? It's not wrong.

In the previous year, we have doubled (double) our income.

And since the word 'class' is a collective noun both 'have' and 'has' can be used here in this sentence, am I right?

The class has done (do) 5 tests this month. .Yes

So both simple past and present perfect are possible after 'in the previous year', but using either present perfect or past simple with 'in the last year' and 'in the last hundred years' would be wrong, right?

The Present Perfect has various uses. Here is a good discussion.
http://learnenglish.britishcouncil.org/en/english-grammar/verbs/present-tense/present-perfect

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Broadly speaking i

Thanks for the link. Yes, I wanted to ask about these examples;
My father knows much because he has traveled a lot. I didn't use past simple, as I thought it would be wrong. If the specific time were mentioned I would use past simple.

The wind has blown off the man's hat, and he can't cathc it. This is happening at the moment of speaking so I used present perfect.

Helen

Hello Nina_nia,
You should say,

In the previous year, we doubled our income.

because when you say "previous", you're talking about a time that's before some other time that you've already talked about. In other words, the time doesn't extend up till "now", but it's well in the past. To use the present perfect here is a mistake.

You should say

Hello, David

Thanks for your reply. Yes it's helpful as always. I made schnitzels a few days ago, but I didn't dare to upload the pictures. Because I didn't want to scare anyone.

Haha! Nobody wants to have schnitzel-induced nightmares. But seriously, all your other food photos were so yummy. And remember, you once inspired me to cook delicious stuffed potatoes for myself. I really appreciated it at the time.