Subjects usually provide "old" information (the background). Trying to pull a "new" focus out of a backgrounded subject creates a mental clash.
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 . Islands
Linguists debate whether these "walls" are built into our mental grammar or caused by how we process information. 1. The Architectural View Subjects usually provide "old" information (the background)
The second gap is inside an "island," but the first "licit" gap makes the whole sentence feel okay to a native speaker. Islands