高一英语单句改错，咋么改？

Question

1、Mr Smith said that he would leave for Europe next Monday.
2、While he walking the dog in the park,he happened to meet an old frieng.
3、That happened that I was reading upstairs when he came in.

Answer 1

next Monday  改为  the next Monday

 While he walking 改为 while  walking   或  while he  was walking   . friend   拼错。

That happened  改为 It happened     , it 形式主语。

Answer 2

1句不认为有错。。。
2. While he was walking 最后是friend吧
3. 应该可以改成。That happend when I was reading upstairs as he came in 吧

Answer 3

1、the next Monday

2、he was walking
3、it happened

Answer 4

1、Mr Smith said that he would leave for Europe next Monday.
宾语从句中要将 next 改为 the next
2、While he walking the dog in the park,he happened to meet an old friend.
while 做主语，要用过去进行时，即 he was walking
3、That happened that I was reading upstairs when he came in.
that 不能作形式主语，只有 it 可以，将 that 改为 it