英文の添削＆解説をお願いします！
1)Mary is studying, so you (not/her/had/interrupt/better).
→had not better interrupt her
2)(be/lot/used/of/there/a/to) fireflies in Japan.
→There used to be a lot of
3)She refused to accept this truth. She could not and [would] not believe that I was mentally and physically handicapped.
1,[Would] you so kindly lend me a hand?
2,He said he [would] arrive by the first train tomorrow morning.
3.No matter how hard we pushed, the door [would] not open.
4,On Sundays he [would] sit for hours doing nothing.
4)I do hope I'm not intruding and I know how busy you [must] be.
1,All living things [must] die.
2,He [must] be tired, because he has been working since eight o'clock this morning.
3,Many people think the ability to understand English is a [must].
4.You [must] be very careful in driving a car to avoid a traffic accident.
１，２は()内の並べ替え、３，４は[ ]内の助動詞に最も近い意味の助動詞を含む英文を選ぶ問題です。
特に３，４はわかりやすく解説してくださると助かります。
よろしくお願いします。
お礼
回答ありがとうございます！！ とても理解しやすいです！