1. Word Usage

2. A / An
The indefinite article a is used before words that do not start with a vowel sound; an is used before words starting with a vowel sound.
Notably, several words begin with the vowel u but are pronounced with an initial consonant sound (often the "yuh" sound):  a union, a unicorn, a United States senator, a useful idea, a uniform. 
Also a few words that begin with consonant h are preceded by an because the h sound is not pronounced:  an honor, an honest man.

What of acronyms?  Again, we go how they are pronounced.  We say "an FBI agent" because we sound the letter F as "ef."  We say "a UN resolution" because we sound the letter U as "you."

3. Any More / Anymore Any more (two words) means no more
We do not have any more work = We have no more work.
Anymore (one word) means now, currently, at this time.
We are not working anymore = We are not working now/at this time.
Both are used with negative constructions.


4. Fewer / Less
Fewer applies to individual countable items (a plural count noun).
Refers to a number
Less applies more general amounts (a noncount noun, always singular). 
Refers to quantity
We have fewer snowflakes and less snow.
We have fewer arguments and less anger.
We have fewer tears and less sorrow.
We have fewer prejudices and less intolerance.

5. Lie / Lay
Lie is an intransitive verb (one that does not take an object), meaning to recline. 
Lie - base form
Lay - past tense
Lain - past participle
Lying - present participle
Lie meaning "to tell an untruth" uses lied for both the past tense and past participle, with lying as the present participle.
Lay is a transitive verb (one that takes an object), meaning to put or to place. 
Lay - base form
Laid - past tense
Laid - past participle
Laying - present participle

6. There / Their / They’re There is an adverb of place:
“No, don't put it here; put it there." 
The other use of there is as an expletive used to introduce a clause or a sentence: 
"There is a monster lurking in the woods.” 
Their is the possessive form of the pronoun they. 
The ideas that they have = their ideas: 
"Their ideas are worth considering." 
The homework belonging to them = their homework: 
"Students should turn in their homework at the beginning of class.”
They're is a contraction for "they are”.
It has no other use or meaning 
"They're going to be late."

7. Than / Then Than is a conjunction used in comparisons:
“Better late than never.”
“He is taller than I am.”
Then is an adverb pertaining to time:
“We said goodbye, and then we went home.”
“Finish your supper; then you can watch TV.”

8. Everyday / Every day
Everyday (one word) is an adjective meaning encountered or used typically or routinely; a synonym is ordinary.
She accidentally wore her everyday coat to church.
Every day (two words) literally has the same meaning as each day.
We have the lowest prices every day.

9. Thru
In formal writing, thru is not an acceptable substitute for through. 
Despite the widespread acceptance of thruway as a variant of throughway, thru is unacceptable and suggests that the writer is too lazy to type or write the three extra letters in the more conventional form of the word.