英文gammar問題

It is not about whether the Dress Casual Day is a one time thing or not.

We use past tense if we want to find out what students thought at that time.

If there is nothing significant happened during that time and now, we can assume that what they thought at that time is the same as what they think now.

We use present tense to talk about things which are true in the past, still true now and probably true in the future.

If the Dress Casual Day happened this year,the verb has a Past Tense.
If not,then the present tense can express actions taking place in perennial=continual every year.condition.
Since time is the concept expressed by the tense of a verb, it is present tense here.Hence word"think" does not use past tense.

如果是今年既Dress Casual Day就要，但系如果呢個Dress Casual Day 系年年都有既話就唔駛