About relative pronouns

[Huhuiowen]

I have taken the test about relative pronouns on which I made two mistakes like following:
1, Your answer was: incorrect
your sentence:
They have a very large house round that there are some lovely gardens.
2, Your answer was: incorrect
your sentence:
I think you will see that this is the best museum which you can find in the town.
I just have a little impression about when I am supposed to use that. If I met the object which refers to the specific one thing, I should use that, am I right? On the other text I can use both of them.
Regarding the 1st item, I think that a very large house can be deemed as the specific one, that's why I chose that.
As regards 2nd, I made mistake by careless.
I am looking forward to your reply.
Sincerely,
Hui

[Kitosdad]

1) They have a very large house around which there are some lovely gardens.


2) I think you will see that this is the best museum that you can find in the town.
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[Huhuiowen]

Yes, I know the right answer.
I just wanna knwo how to use both of them.
Thanks for your reply.