- This lesson is about essential and non-essential adjective clauses. Here is A. The boy, who had attended all the lectures, lent me his study notes. Here is B.

The boy who had attended all the lectures lent me his study notes. Look at the commas. That is the only difference. Two commas look like a small difference. They are not.

Look at A. We are talking about one specific boy and you know which boy I mean. His name is Jim. I could also say Jim, who had attended all the lectures, lent me his study notes.

Now take the clause out and take its commas out with it. The boy lent me his study notes. We still know exactly which boy. It is still Jim. Nothing important was lost. So, this clause is non-essential.

It is extra information. Now let's look at B. There are no commas this time. We are in a classroom of boys. Bob is getting an F, Paul's getting a D. Jim went to every lecture, so he is getting an A.

Which boy lent me his notes? The one who attended all the lectures, Jim. Now take the clause out again. The boy lent me his study notes. Which boy? We cannot tell.

It could be Bob and his notes will not help you. Losing the clause loses the meaning, so this clause is essential. Let's put the two ideas together. Here is the test. Take the clause out and read the sentence again.

If you cannot tell which one the clause is essential and it has no commas, if you can still tell the clause is non-essential and it sits between commas. Let's practice with a new sentence. Here is a sentence with no commas.

The man whom we had invited to dinner did not show. Whom is the object form of who? There are many men. Which one did not show? Only the clause tells you, so it is essential. Now watch what commas do.

Now look at this. The man, whom we had once invited to dinner, was moving to Oregon. The word is still whom, but now there are commas. The big news here is that he is moving to Oregon. The dinner was a long time ago and it is extra information.

Take the clause out. The man was moving to Oregon. We still know which man we mean. So, this clause is non-essential. Look at the two sentences about the man. The word is whom in both, only the commas changed.

Now let's look at the words themselves. Now which word goes inside the clause? For a person, it is who or whom either way, and the commas carry the difference. For a thing, the word changes. Essential matches up with that and non-essential matches up with which.

Let's try one with a thing. The ping pong table, which is upstairs, is broken. I do not have 20 ping pong tables. I have one, so where it sits is extra information. Take it out. The ping pong table is broken.

So, this clause is non-essential. Bring your paddle. You will not get to play. Now compare. The ping pong table that is upstairs is broken. No commas, and the word is that.

So, I have more than one table and this clause tells you which one. This clause is essential. Go play on the one in the garage. Now let's put it all together. Look at all four together.

Without commas, essential, with commas, non-essential. For the boy, the word is who both times. For the table, it changes. That goes without the commas and which goes with them. The commas are always the signal. Now here is the rule.

No commas means the clause is essential. It tells you which one. Commas mean the clause is non-essential. You can drop it. Read the commas first. They will tell you every time, that and which come up again next lesson.

Once you see the commas, the rest gets easier.