1. I saw something like this when I watched/read (name of TV programs, title of news article, title of video watched like documentaries)
What I know is

I saw something like this when I watched Pawn Stars.

What I know is that this is a rare sword made to honor heroes in Scotland. There are not many of them, but to ensure that it is an original one, you should contact an expert and what is more take extra care of it.

Explanation:

In this case, we mention a TV show. We relate what we have seen and learned in the tv show with a real life experience where we implement the little knowledge that we have about an object in a situation.

We could have also learned some specific information from a book, video, news article, or any other source. The interesting about what we have learned is that we can implement it in a real situation, no matter where we got the information.

a complex sentence consists of an independent clause and one or more dependent clauses.  

to write complex sentences, we must use subordinating conjunctions.  

all the complex sentences begin with a subordinating conjunction and when it occurs, a comma must be place at the end of the dependent clause. on the contrary, if the sentence begins with an independent clause, then no comma is used.

after  

although  

as  

because  

before  

if  

since  

when  

for example,  

after she won the competition, she became very popular.  

although my father was hurt, he continued fighting for his rights.  

as all of you got a ten, you will be able to leave early today.  

before he went to the war, he asked his parents to bless him.  

if it is cold, i won’t go out.  

since you didn’t bring the homework, you will get low grades.  

when it is hot in the town, everyone goes to the beach.