English and ESL Instructor
I opened the door there was nobody in the room.
The first clause, I opened the door, is an independent clause, as is There was nobody in the room. In other words, both can be sentences by themselves. However, there is no punctuation in between. The proper punctuation mark that we need in between these clauses is a period:
I opened the door. There was nobody in the room.
Alternatively, we can use a comma and conjunction:
I opened the door, but there was nobody in the room.