tests by marc10 with the online test builder
(12 tests)
Ranked by: number of candidates
1. A, an, the (5583 candidates) 
2. Simple past, present perfect, past perfect (2667 candidates)
3. Who, whom, which (1365 candidates)
4. Future simple or continuous (1196 candidates)
5. In, At, On (927 candidates)
6. Possessive case (859 candidates)
7. Pronunciation of -ed at the end of the verbs (830 candidates)
8. Comparative or superlative (762 candidates)
9. Verbs of opinion (754 candidates)
10. Animals (664 candidates) 
11. Opinions : neither, so (658 candidates)
12. Present simple or continuous (598 candidates)
(12 tests) Ranked by: number of candidates
1.
A, an, the (5583 candidates) 2.
Simple past, present perfect, past perfect (2667 candidates) 3.
Who, whom, which (1365 candidates) 4.
Future simple or continuous (1196 candidates) 5.
In, At, On (927 candidates) 6.
Possessive case (859 candidates) 7.
Pronunciation of -ed at the end of the verbs (830 candidates) 8.
Comparative or superlative (762 candidates) 9.
Verbs of opinion (754 candidates) 10.
Animals (664 candidates) 11.
Opinions : neither, so (658 candidates) 12.
Present simple or continuous (598 candidates) 