Can I say:
I made something you never think is possible.
The focus here is not the tense.
Here is a similar sentence:
I can build stuffs that you never imagine can be built out of wood.
The of the word 'that', the way I was taught, is that the sentence after it is complete with the word before it. Here is an example:
I want that apple that no one touches.
The sentence after 'that' along with the word before it is:
No one touches that apple.
OK. Now, let's go back to the first example. The sentence after 'that' is:
You never imagine can be built out of wood.
In order for it to be complete, you need to insert the word 'stuffs', which is right before 'that'. But PAY ATTENTION, this time, we are inserting the word in the middle of the sentence, not at the end:
You never imagine stuffs can be built out of wood.
Now, the question is can you insert that word in the middle.