|Knots? They restrict--just as restrictive clauses tighten the meanings|
1. What is a clause? Not Santa (sorry for the tired homonymal pun). If you said it is a group of words with a subject (the person doing the action) and a verb (the action) you are correct. An example of a clause is: When I balance my checkbook. We have a subject (I) and a verb (balance). A clause need not be a sentence (nor a fact, because I never balance the stupid thing).
2. What is a restrictive clause? (And yes, I'm having a bad pun day with my allusion to that infamous Indian book, but I don't think it was an S & M book, so restrictive may not work. Restrictive is essential to the meaning of the sentence. For example, The test that I will give after this lesson will be a cinch.
In this example, I will give... is essential for your understanding, ergo restrictive.
3. What is a non-restrictive clause? Obviously, it's one you don't need. The test, which no one will take or pass, will be a cinch. We don't need the junk between the commas.
Which is sutured with a comma. Which is knot?
a.) A studebaker I tied for you will keep you safe.
b.) A studebaker some call a Swiss harness is used for climbing
Answer: You need the comma in example B.
Ready, set: test next time.