✔ 最佳答案
1. Of the three sentences you wrote, the following two are correct in grammar with different meaning.
He has some thing to do.
Note:
~ "some" is determiner 某些/個〔未知或不確指的人或物〕
~ "thing" must be singular
He has some of the things to do.
Note:
~ "some" 一些 is a pronoun here
~ "the things" are something that was mentioned before.
2. 點解要加of?
All the students
"all" is determiner 限定詞 here, meaning "the whole number of" 全部
All of the students
"all" is a pronoun here.
You use "all of" because "the students" are those you have mentioned before.
Preposition "of" has several meanings. You may refer to the dictionaries for its different usages.
3. which as a pronoun 哪一個; 哪些〔用於選擇疑問句中〕
正確的語法： (future tense)
When will your team be free for discussion? (一般性的提問) day and time
Which of the weekdays will your team be free for discussion? (可選擇的提問) 週一至週五，哪一日?
What time (of the day) is your team free for discussion? ("is" for future tense)
Which time do you prefer to have the discussion with us, morning or afternoon?
Combined question:
Which of the weekdays and what time will your team be free for discussion?
free 空閒的; 不忙的 (for something) 係特定用法
