This one is wrong. It should be: 電話がとおいの
This one is wrong. It should be: つうじないの
This is right. The translation is: "The line was busy, so I was upset."
This one is wrong. It should be: 読みにくいの
This one is wrong. 何も must be connected with a negative.
This is right. The translation is: "I did not study anything."
This one is wrong. It should be: ふべんな.
This is right. The translation is: "Because it was broken, I didn't use it."
This is right. The translation is: "Because it may not be good, I didn't use it."
This one is wrong. 何本 is an extent expression. No particle is necessary.
This one is wrong. 何本 is an extent expression. No particle is necessary.
This is right. The translation is: "How many did you buy?"
This is right. The translation is: "Did you buy many (long cylindrical things)?"
This one is wrong. 何本 is an extent expression. No particle is necessary.
This is right. The translation is: "My younger brother taught." (Humble polite)
This one is wrong. なさいました is honorific polite, and cannot be used to refer to someone in your in-group.
This is right. The translation is: "My younger brother does (something)." (Humble polite)
This is right. The translation is: "Didn't my younger brother borrow (it)?"
This is right. The translation is: "S/he may have been free."
This is right. The translation is: "It may be a pretty car."
This one is wrong. Before かもしれません only direct style occurrs. It should be: "聞こえなかったかもしれませんよ。"
This one is wrong. It should be: "すうじによわいかもしれませんよ。"