Recently I read the following dialogue:
A: 今天有转校生要来吗？
B: 听说是这样，好象是临时决定的，所以才没有及时通知我们。
As I was trying to make out what they were saying, I noticed that 临时 and 及时 have similar meanings. The former seems to mean "at the instant something happens" or "ad hoc" while the latter seems to mean "promptly" or "without delay."
I have a feeling that in the second sentence, replacing "好象是临时决定的" with "好象是及时决定的" would also work.
What's the difference between them, and when should I use one over the other?
When you use "临时", your point is "wasn't part of the plan".
When you use "及时", your point is "rapidly/promptly".
So if you replacing "好象是临时决定的" with "好象是及时决定的", it also works but the point will become to "They made the decision promptly" from "They made the decision that wasn't the part of the plan (of the meeting)".
Of cource "临时" has other meanings, such as "temporarily", which means a temporary solution which will be replaced by the formal one later on.
Cannot use them interchangeably. '及时' is just 'in time/without delay', '临时' is unplanned, not always similar to 'immediately'.
临时 means unplanned
他临时改变了计划,He changes the plan temporally.
It means he does not stick with the origin plan, and sometimes the newest plan is often bad.
及时 means timely
他及时改变了计划 He changes his plan timely.
It means thanks to his wisdom, he changes his plan or something bad will happen.
临时 (臨時) = Temporary
及时 (及時) = in time
