Theory:

Words that have near similar meanings but cannot be used interchangeably: They are similar words but have a minor difference in meaning and change according to context and sentence formation. Some words may sound the same, but have entirely different meaning also.
I. Affect - effect:
  
Affect means make an effect/difference to (verb).
 
Effect means a result or a consequence of an action (noun).
Example:
  1. The chillness affected my throat.
          But, "The chillness effected my throat" is wrong usage.
 
2. The harmful effects of pollution are numerous.
  
II. All - every:
  
All — a determiner that denotes the whole quantity or group.
 
Every — also a determiner - refers to individual members of the group.
Example:
  1. The meeting starts at 8; all are welcome.
          But, "The meeting starts at 8; every are welcome" is wrong usage.
  
2. Laws are applicable to every citizen of the country.
  
III. Anymore - any more:
  
Anymore — any longer (adverb).
 
Any more — question form of some more.
Example:
  1. She does not live here anymore.
  2. Do you need any more tea?
Important!
For a list of commonly confused words, click here.
Commonly confused words from the lesson "The Three Questions":
 
No.
Word from
 the lesson
Commonly
confused words
Sentence from the lesson
1
Right
Rite
If he always knew the right time to begin everything; 
2
Might
Mite
He would never fail in anything he might undertake.
3
Laid
Lied
There were some things which could not wait to be laid before a council.
4
Whether
Weather
One had to decide whether to undertake them or not.
5
Science
Signs
The most important thing in the world was science.
6
Cell
Sell
So the king put on simple clothes and, before reaching the hermit’s cell, dismounted from his horse.
7
Weak
Week
The hermit was frail and weak.
8
Stuck
Struck
The king at last stuck the spade into the ground.
9
Wood
Would
The king turned round and saw a bearded man come running out of the wood.
10
Ceased
Seized
When at last the blood ceased flowing, the man revived and asked for something to drink.
11
Sun
Son
Meanwhile the sun had set, and it had become cool.
12
Peace
Piece
The king was very glad to have made peace with his enemy.
13
Knees
Niece
The hermit was outside on his knees.
14
Pray
Prey
I pray you to answer my questions.
15
Council
Counsel
He should have a council of wise men who would help him.