Use that when the words following it are necessary to identify the word that refers to.
Example: “The river that flows by my door is rising.”
You cannot remove the that clause, because you wouldn’t know which river is being referred to. The sentence would simply read: “The river is rising.”
Use which when the words following it are not necessary to identify the word it refers to.
Example: “The Indian River, which flows by my town, is rising.”
You can remove the which clause, because you would know which river is being referred to – the Indian River. The sentence would read: “The Indian River is rising.” Note: When using which, use commas to separate the clause. When using that, don’t use commas.
When using the adjective so-called to describe something that is falsely or improperly so named, do not put the word or phrase it describes in quotation marks. INCORRECT: The so-called “leader” of our group is really a follower. CORRECT: The so-called river that runs between Cocoa and Merritt Island is technically a lagoon.
Be careful not to confuse words in your writing. One often sees onboard used where aboard is the proper word. INCORRECT: We’re happy to welcome Smithers onboard as our new Chief Fun Officer. Onboard means “carried within or occurring aboard a vehicle (as a satellite or an automobile).” For example, an onboard computer. CORRECT: We’re happy to welcome Smithers aboard as our new Chief Fun Officer. Aboard means “in or into a group, association, or organization.”
Other common confusions (check a dictionary if in doubt):
- compliment and complement: With the i it means “an expression of esteem”; with the e it means “something that completes.”
- eminent and imminent: The e word means “standing out”; the i word (and note the double-m) means “ready to happen.”
- under way and underway: The one-word form is not to be used after a verb, i.e., the one-word form is only an adjective. Incorrect: We got underway at noon. Correct: We got under way at noon. The conference is under way as we speak. The crew prepared for the underway refueling scheduled at dawn.