Can you tell me the difference in meaning between these two sentences?
1. Olya entered the chapel and approached the altar where Dima knelt with his wife.
2. Olya entered the chapel and approached the altar, where Dima knelt with his wife.
The only difference is the comma—what does that little comma do? It separates the sentence into two portions, which serves to inform the reader that the second portion of the sentence is adding some extra information. The comma, in essence, informs the reader that the second part of the sentence is not necessary to understand the first part.
Without the comma, the sentence implies that that last portion of the sentence (“where Dima knelt with his wife”) is clarifying the first part, that the sentence wouldn’t be complete without it. If that last portion of the sentence is necessary, then that implies that the “altar” that was mentioned needs to be distinguished from some other altar in the area.
In other words, here is what each of our example sentences above is really saying:
1. Olya entered the chapel. There were several altars, and she approached the one where Dima knelt with his wife.
2. Olya entered the chapel and approached the only altar. Dima knelt beside the altar with his wife.
See the difference? Without the comma, the implication is that the second portion of the sentence is necessary to clarify the first portion. With the comma, the implication is that the second portion of the sentence is extra information which isn’t needed to understand the first portion.
Imagine that you and a friend are headed to the movies, when you get a call that you need to stop and meet a secret agent to receive a package related to your next secret agent mission. (We’re all imagining that you have a very interesting life.) You tell your friend that you have to stop to meet this agent at the park.
Now, how you phrase this statement would depend on how many parks are in the area, right? If there were only one park in the whole town, then you might simply say:
“We have to stop at the park.”
Since there’s only one park in the area, your friend would know what you mean. However, if there were several parks in the area, you would need to add additional information to clarify which park you meant:
“We have to stop at the park where you met my sister.”
See how it works? The phrase “where you met my sister” serves as clarification—without that information, it would not be clear which park you meant. Since the information is necessary, it should not be separated from the noun it clarifies (park) by a comma. If you did so, the sentence would read strangely:
“We have to stop at the park, where you met my sister.”
Placing a comma here implies that you are adding extra information, which in this case seems rather nonsensical. Why would you mention, apropos of nothing, that your friend met your sister in the park? Your friend would know that. With the comma, the sentence essentially means this:
“We have to stop at the park. By the way, you met my sister at this park.”
That just reads like particularly awkward maid-and-butler dialog. Imagine your friend looking at you in confusion, and saying, “Yeah, I know that. Why are you telling me?”
Remember: clarifying information should usually not be separated from the word it clarifies by a comma.