To understand the grammar of the sentence, you must break it       into parts. 

-- Do you think it's a good idea to make friends with your students?-- ______, I do. I think it's a great idea.

There are citizens who support graffiti, saying it’s a welcome addition to _________ dull street.

I recognized the man _______ he came into the room.

The newly­learnt words will be impressed on us only if they are ________used in everyday communication.