Hi Heidi, The relative clause "which is the capital of China" is not an extraneous piece of information in this sentence. In fact, it defines what Beijing is — the capital city of China. If a relative clause is defining (or restrictive) because it provides essential information about the noun to which it refers, we do not put a comma before the relative pronoun. On the other hand, if your sentence were different (e.g. "I went to Beijing, which is the capital of China, for a symposium"), the main idea would be your travelling to Beijing to attend a symposium. The fact that Beijing is a capital city is an additional piece of information. A simple test would be to remove the additional piece of information and see if the meaning of the sentence remains intact. In this case, we would end up with "I went to Beijing for a symposium." The meaning remains intact; thus, it is a non-restrictive relative clause. Therefore, we use commas. I hope this helps you.

Yes, you do. For the use of "which", in this case, is for a nonrestrictive clause (i.e. the capital of China). By this I mean that, if you take out that part of the sentence, it won't change the meaning of it. It'll just make it less informative. You use "which" to add additional information about the previous part of the sentence. Take for example: "My cat Gatticus, which happens to be two years old, meows whenever he's hungry".

I wouldn't put a comma there myself. Adding a comma adds a pause which makes the sentence sound a little unnatural to my ears. A good test if a comma is needed is read the sentence to yourself. If you want a pause there, add a comma. Otherwise, if you want the sentence to flow with no pause then don't add a comma.

You don't need a comma there. If you wanted to make a longer sentence you would need a comma. Example: I went to Beijing which is the capital of China. Or I went to Beijing, which is the capital of China, to visit my family.