I’ll answer two questions at the same time. Do you use that or which? Do you need commas with that and which?
To answer both questions, you need to understand restrictive and non-restrictive phrases and clauses. Here are the answers in brief.
1. Use which and commas with non-restrictive phrases and clauses.
2. Use that and no commas with restrictive phrases and clauses.
Now, let find out why.
Restrictive phrases and clauses: A restrictive phrase or clause points out which thing you are writing about.
Let’s say you have four filing cabinets in your office and that all but one cabinet is locked. The unlocked cabinet is the one next to the window. You need someone to come and lock the cabinet because you don’t have a key.
You decide to send an e-mail to the maintenance office. You need to tell the maintenance officer which cabinet is unlocked. You correctly write this statement:
“Please come to my office as soon as possible and lock the cabinet that is next to the window.” (The restrictive phrase is underlined.) Continue reading
(From Concise Guide to Technical and Academic Writing.)
An expert academic or technical writer needs only a few basic sentence patterns to produce easy-to-understand writing. Each of the sentence patterns below will result in clear academic or technical writing. However, do not use any one pattern more than twice in a row to prevent the writing from sounding repetitive and boring. Also, use the more complex sentence patterns less frequently. They are more challenging for the reader and may make the writing overall more complex than necessary.
All effective sentence patterns start with the Subject-Verb-Object (S-V-O) sentence structure. Optional components are additional S-V-O structures and descriptive words, phrases, and clauses (D), which can be placed in various locations.
In the samples below, the subjects are underlined, and the main verbs are in italics.
1. Simple sentence (S-V-O): A simple sentence has one subject–verb pair. It starts with the subject (or an adjective and the subject). The subject is immediately followed by the verb (or an adverb and the verb). A simple sentence may contain an object.
Example 1: The computer desktop provides access to your files. Continue reading