A & AN: Vile errors
Guideline. Use a before any word that begins with a consonant sound. Use an before any word that begins with a vowel sound. Easy enough. We'll dispense with the obvious examples and go right to the ones we see missed every day: sounds that begin with consonants but are pronounced as though they start with vowels - and vice versa.
- My company has an HMO plan for medical problems.
- The proposal came to us as an MS Word document.
- This is a historic occasion. [In UK English, though, historic is usually preceded with "an" because of the softer pronunciation of the "h" in "historic."]
AS, BECAUSE & SINCE: Ongoing problem
Guideline. Use because or since when you might otherwise say "the reason being." Use as when you would otherwise use "during."
- I didn't hear any of the commotion that went on last night because [the reason being] I was fast asleep.
- I didn't get a chance to see any of the finalists as [during the time] they were marching by very quickly.
- I didn't get a chance to see any of the finalists because [the reason being] they were marching by very quickly.
DIFFERENT FROM & DIFFERENT THAN: Marked distinctions
Guideline. Favor different from in most cases, but keep in mind that different than is now considered acceptable in many situations.
- Morris is different from the way I imagined he would be, based on his photograph.
- The situation is a lot different today than it was just yesterday during the slalom competition. (The alternative here would be wordier: "The situation is a lot different today from the way it was yesterday during the slalom competition.")
DUE TO, BECAUSE OF & OWING TO: Just cause
Guideline. Use these phrases interchangeably, but be prepared to defend your use of due to when it follows an action verb.
A closer look. Due to fell into disrespect in the early 1900s, when grammarians decided it was an adjective and therefore could not modify an action verb. Some usage authorities still hold that position, but the modern view is that due to is a prepositional phrase and can therefore be used wherever you feel like using it.
- My sunburn today is due to my falling asleep in the noonday sun yesterday. (Due to is standard here because it follows a linking verb, is.)
- I didn't sleep a wink last night due to [or because of or owing to] the noise created by the flamenco dancers who live above me.
Source: Grammar for Smart People by Barry Tarshis.