If you come to China, I host you.

Although zero conditional means that the conditional phrase is true now and always (in other words, the result is certain), but in the context of your sentence, it sounds odd even though some may argue it is technically correct. Maybe the following sentence works better - "if you don't ever travel, you do not need to get a passport."

(First conditional)

The first conditional represents something that is most likely going to happen. Almost certain.

If you came to China, I would host you.

Second conditional describes a situation that is imaginary and most likely will not happen. Although the verb tenses in your sentence are correct, the sentence really does not fall under the category of second conditional. An example second conditional sentence would be - if you did not spend all your money on gambling, you would have enough money to buy a house. So, the act of buying a house is very uncertain because the money is already spent.

If you had been to China, I would have hosted you.

Third conditional sentences are used to explain that present circumstances would be different if something different had happened in the past. You could have changed your sentence like that - "If you had come to China before COVID 19, I would have hosted you."

If I study hard, I will get good grades.

This sentence will sound even more elegant if you wrote "If I study hard now, I will get good grades."

If I studied hard, I would get good grades.

Again, you need to use the second conditional in a situation where the outcome is most likely NOT going to happen. Maybe write something like this - "If I studied hard, I could become as rich as Bill Gates." Please note, when using the second conditional, you could also use words like "would, might, were, etc."

If I had studied harder, I would have got good grades.earned better grades than Amy.

Third condition - your present situation of getting a worse grade than Amy is caused by the fact that you did not study hard enough in the past.

The above example sentences are how I understanood conditional sentences to be.

You are almost there!! English is really tough and sometimes the rules do not make sense. It is hard to get the proper nuance without immersing yourself in the language on a daily basis. I'm struggling with my french, so I can super sympathize with you.

Thank you for your teachingguidance.

If you come to China, I will host you.

The action of coming occurs before the action of hosting, so we need to write “I will host.”

If I study hard, I will get good grades.

Without any other context, I would personally write “will get,” but the original sentence can also be acceptable, depending on the situation.

If I had studied hard, I would have gotten good grades.