Extracting from a subject might simply be too mentally taxing for the brain to process in real-time. Exceptions and "Parasitic" Gaps
Subjects usually provide "old" information (the background). Trying to pull a "new" focus out of a backgrounded subject creates a mental clash.
Once a subject moves to its final position, its internal structure is "frozen" and cannot be accessed. Islands
Many generative grammarians argue that islands are a result of the of human language.
Not all subject islands are equally strong. Some violations become acceptable if they are "saved" by a second gap in the sentence, known as a . Extracting from a subject might simply be too
Modern theories suggest certain phrases are "phases" that become invisible to the rest of the sentence once completed. 2. The Information Structure View
Some researchers suggest the problem isn't grammar, but . Once a subject moves to its final position,
"Who did you see [a picture of ___]?" (The phrase is the object).