I think the biggest mall I've ever visited is premium outlet in Siheung Gyunggido it is very big I will go to there next weekend with my family.

Here's your corrected composition, Yale!  Many thanks for sending it. Please take a look at the few revisions and alternative expressions provided. 

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                                             Describe the biggest mall you've ever visited.

I think the biggest mall I've ever visited is premium outlet in Siheung Gyunggido it is very big I will go to there next weekend with my family.

>> I think the biggest mall I've ever visited is the premium outlet in Siheung Gyunggido. It is huge and I will go there again next weekend with my family.