tests by marc10 with the online test builder
(12 tests)
Ranked by: number of candidates
1. A, an, the (5633 candidates) 
2. Simple past, present perfect, past perfect (2764 candidates)
3. Who, whom, which (1384 candidates)
4. Future simple or continuous (1211 candidates)
5. In, At, On (929 candidates)
6. Possessive case (874 candidates)
7. Pronunciation of -ed at the end of the verbs (835 candidates)
8. Comparative or superlative (763 candidates)
9. Verbs of opinion (759 candidates)
10. Animals (667 candidates) 
11. Opinions : neither, so (660 candidates)
12. Present simple or continuous (598 candidates)
(12 tests) Ranked by: number of candidates
1.
A, an, the (5633 candidates) 2.
Simple past, present perfect, past perfect (2764 candidates) 3.
Who, whom, which (1384 candidates) 4.
Future simple or continuous (1211 candidates) 5.
In, At, On (929 candidates) 6.
Possessive case (874 candidates) 7.
Pronunciation of -ed at the end of the verbs (835 candidates) 8.
Comparative or superlative (763 candidates) 9.
Verbs of opinion (759 candidates) 10.
Animals (667 candidates) 11.
Opinions : neither, so (660 candidates) 12.
Present simple or continuous (598 candidates) 