1) Who vs. Whom
a) Who is used as the subject of a sentence.
• The teacher who gave the lecture is very experienced. (The teacher is the subject)
b) Whom is used as the object (often after a preposition or a verb).
• The student whom I helped passed the exam. (The pronoun represents the object of 'helped')
• To whom did you give the book? (After a preposition)
2) Which vs. That
a) Which is used for non-essential information (with commas).
• The Eiffel Tower, which is in Paris, is a famous landmark. (The information can be removed without changing the main meaning)
b) That is used for essential information (without commas).
• The book that you recommended was amazing. (Without this information, we wouldn’t know which book)
3) Whose vs. Who’s
a) Whose shows possession.
• The girl whose dog ran away was very sad. (The dog belongs to the girl)
b) Who’s is the contraction of ' who is ' or ' who has. '
• Who’s coming to the party? (= Who is coming?)
• Who’s seen my keys? (= Who has seen?)
4) Where vs. When vs. Why
a) Where is used for places.
• This is the house where I grew up.
b) When is used for time.
• I remember the day when we first met.
c) Why is used for reasons.
• I don’t know the reason why he left.
5) What vs. Which
a) What is used when there is a broad choice or no restrictions.
• What is your favorite color? (Among all possible colors)
b) Which is used when the choice is limited.
• Which dress should I wear? (There are a few specific options)