... to find out whether this virus causes the whole immune system... / whether he has ever come back
1.
The researchers are eager to find out whether this virus causes the whole immune system to break down.
→Is this sentence correct? I find that “whether this virus causes…” is a little odd, and it should be “whether this virus can or could cause…”. Am I right?

2.
His mother couldn’t remember whether he has ever come back after his father died.
→Is this sentence correct? Is the word “ever” used correctly here? Can I say "can't" instead of "couldn't"? Are there any differences between them?

Thanks!

Question

... to find out whether this virus causes the whole immune system... / whether he has ever come back
1.
The researchers are eager to find out whether this virus causes the whole immune system to break down.
→Is this sentence correct? I find that “whether this virus causes…” is a little odd, and it should be “whether this virus can or could cause…”. Am I right?

2.
His mother couldn’t remember whether he has ever come back after his father died.
→Is this sentence correct? Is the word “ever” used correctly here? Can I say "can't" instead of "couldn't"? Are there any differences between them?

Thanks!

Answer

1. It's 'can cause' since it's still suggesting a possibility, and as personal preference, you can add "or not" after "whether." Or not. HAHA

2. 'ever' is fine, while "can't and couldn't" depends on the time when the action was done. Can't is present, while couldn't is past.This is usually determined by context, so the choice is dependent on previous sentences, like if the son was retelling the story then it would be couldn't but if it is while mother and son is talking, then it is can't. Again, you can use "or not," or not. Hihi.