...millions of people were watching the royal../"historic/historical"sites/an English native speaker
1.
Hundreds of millions of people were watching the royal wedding of Prince William and Kate Middleton on TV when it took place in 2011.
→Is this sentence correct?

2.
What makes English Heritage well known is it guards over 400 "historic/historical"sites.
→Is this sentence correct? Which is correct and more common saying in the quotes?

3.
a. He is a native speaker of English.
b. He is an English native speaker.
→I know sentence a is correct. But is sentence 1 correct, too? And, how about just "native", not "native speaker"?

Thanks!

Question

...millions of people were watching the royal../"historic/historical"sites/an English native speaker
1.
Hundreds of millions of people were watching the royal wedding of Prince William and Kate Middleton on TV when it took place in 2011.
→Is this sentence correct?

2.
What makes English Heritage well known is it guards over 400 "historic/historical"sites.
→Is this sentence correct? Which is correct and more common saying in the quotes?

3.
a. He is a native speaker of English.
b. He is an English native speaker.
→I know sentence a is correct. But is sentence 1 correct, too? And, how about just "native", not "native speaker"?

Thanks!

Answer

'He is an English native speaker' is something that native speakers do not say. It is weird and unnatural. 'English' is modifying 'native speaker' and thus the meaning is 'he is a native speaker from England.' Of what language though? It's just strange.

'He is a native English speaker' on the other hand is perfect and what is overwhelmingly preferred (to mean that the person's native language is English.)

Answer

3. Both sentences are correct.
If you say to someone "He is an english native", it would be a very odd way of you to tell me that a person was born and raised in england. 
When you add the "speaker", it clarifies that you're talking about language and telling me that he speaks english fluently because he was born in england.

Answer

2. If you add "that" between is and it, the sentence will be correct with both historic and historical.
Choosing between the two is only a matter of which makes your sentence sound better.

"Guards over" is not a commonly used phrase, it reveals that the speaker is not a native speaker - a better way of phrasing it might be that it "encompasses" or "takes care of " more than 400 historical sites.

Answer

1. "hundreds of millions" is more than 100.000.000 million people - a very very large amount of people, it would be more appropriate to say "hundreds of thousands of people" which would mean that up to a million people were watching.
Otherwise the sentence works, even if it is a little stiff :) Potentially you could remove the "on TV" bit.