tests by marc10 with the online test builder
(12 tests)
Ranked by: number of candidates
1. A, an, the (5513 candidates) 
2. Simple past, present perfect, past perfect (2507 candidates)
3. Who, whom, which (1330 candidates)
4. Future simple or continuous (1187 candidates)
5. In, At, On (922 candidates)
6. Possessive case (855 candidates)
7. Pronunciation of -ed at the end of the verbs (826 candidates)
8. Comparative or superlative (761 candidates)
9. Verbs of opinion (753 candidates)
10. Animals (664 candidates) 
11. Opinions : neither, so (651 candidates)
12. Present simple or continuous (598 candidates)
(12 tests) Ranked by: number of candidates
1.
A, an, the (5513 candidates) 2.
Simple past, present perfect, past perfect (2507 candidates) 3.
Who, whom, which (1330 candidates) 4.
Future simple or continuous (1187 candidates) 5.
In, At, On (922 candidates) 6.
Possessive case (855 candidates) 7.
Pronunciation of -ed at the end of the verbs (826 candidates) 8.
Comparative or superlative (761 candidates) 9.
Verbs of opinion (753 candidates) 10.
Animals (664 candidates) 11.
Opinions : neither, so (651 candidates) 12.
Present simple or continuous (598 candidates) 