1. I searched for my glasses, but I couldn't find them.
2. I searched for my glasses, but I couldn't find it.
(Which one is grammatical?)
3. The electric fan was moving from left to right.
4. The electric fan was moving from the left to the right.
(Which one is grammatical? Do you have some more useful expressions?)
1 is correct; 2 is not.
3 is correct; 4 is not.