Ranked by: number of candidates
1.
Can / could/ may/ might (382029 candidates) 
2.
Possessive adjectives and pronouns (325657 candidates)
3.
Pronouns (316620 candidates)
4.
Must or Have to? (289602 candidates) 
5.
Used to/would (238952 candidates) 
6.
Questions with who what where...... (156620 candidates)
7.
Present continuous (146024 candidates)
8.
How much/how many (142204 candidates) 
9.
Place of adverbs (106856 candidates) 
10.
Past tense in the interrogative form (103659 candidates)
11.
Past simple tense (91553 candidates)
12.
Possessive adjectives (91491 candidates)
13.
Past tense in the negative form (82555 candidates)
14.
Adverbs : always, never, usually... (76034 candidates) 
15.
Must/ Mustn't (75588 candidates) 
16.
Adjectives (67943 candidates) 
17.
Exclamation (65656 candidates) 
18.
Present perfect in the interrogative form (65599 candidates)
19.
Defining words: who/which/whose (61099 candidates)
20.
So / such (58729 candidates)
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