Which hobbies are the most popular in your country? 

In our country most popular his playing soccer, and soccer is the biggest league in the world.​ --> In our country, the most popular hobby is playing soccer and soccer has the biggest league in the world. => Corrected : The sentence is corrected grammatically.

​Someone's knocking on the door.  ✔​

They hugged and said goodbye.  ✔​

He will tie the pieces together. --> He will bind the pieces together.  => Another way: Your sentence is okay and this is another way of saying it.

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