If he should fail,he would kill himself.为什么用should ,不是should

What if you should fail? 怎么回答? what he should do why should he? what should he does? If he should come here tomorrow,I would talk to him. He does not know if he should bring a present.中的bring为什么不用ta If he took Flight 307,he () on time.为什么填arrived ,而不是should+动 英文should咋不是would? We should like to ---------- some suggestions which would he I should come too,if I had time.中的should为什么不用would? If he ( )help me,I should be very grateful to you.为什么只能填shou 英语翻译Fail they surly would should accident or vigilance appri