He was afraid whether/that his father... // a student has to pay for the dormitory for a semester
1.
a. He was afraid whether his father knew about his decision of leaving home and joining the army.
b. He was afraid that his father knew about his decision of leaving home and joining the army.
→Are the two sentences both correct? My teacher said that sentence a is wrong because the word "whether" couldn’t be used after “afraid” and the correct word here is “that”, i.e. sentence b. Really?

2.
"The freshman wants to know how much money a student has to pay for the dormitory for a semester."
→Is this sentence correct?

Thanks!

Question

He was afraid whether/that his father... // a student has to pay for the dormitory for a semester
1.
a. He was afraid whether his father knew about his decision of leaving home and joining the army.
b. He was afraid that his father knew about his decision of leaving home and joining the army.
→Are the two sentences both correct? My teacher said that sentence a is wrong because the word "whether" couldn’t be used after “afraid” and the correct word here is “that”, i.e. sentence b. Really?

2.
"The freshman wants to know how much money a student has to pay for the dormitory for a semester."
→Is this sentence correct?

Thanks!

Answer

1.  I think the best English would be:

"He was afraid that his father might know about his decision to leave home and join the army."

Note the "might" and the "decision to …".

"He was afraid whether …" is definitely wrong.

2.  The freshman wants to know how much it costs to stay in a dorm room per semester.

Answer

I'm a native English speaker and I think sentence 1.a is fine - but it depends on context
I might write it: "he was afraid of whether or not his father..."
Although the word is not necessary at all: "he was afraid his father.."

Sentence 2 seems ok as well, but be careful of tense changing - wants vs. wanted etc. Also dependent on context.