1. I will leave at exactly five o'clock.

2. I will leave at five o'clock exactly.
3. I will leave exactly at five o'clock.

4. I will leave at precisely five o'clock.
5. I will leave at five o'clock precisely.
6. I will leave precisely at five o'clock

7. I will leave at promptly five o'clock.
8. I will leave at five o'clock promptly.
9. I will leave promptly at five o'clock

10. I will leave at just five o'clock.
11. I will leave at five o'clock just.
12. I will leave just at five o'clock.
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Thank you for your help. I think we can use 'exactly', 'promptly', 'precisely', and 'just' to say something happened on time. By the way, how about the word orders? Which sentences are grammatical in terms of the word order?

In terms of word order, all of the sentences you provided are grammatically correct. The position of the adverb (exactly, precisely, promptly, just) in the sentence can vary, as long as it is placed before the time phrase "at five o'clock." Here's a breakdown of the word order for each sentence:

1. I will leave at exactly five o'clock. (adverb before the time phrase)
2. I will leave at five o'clock exactly. (adverb after the time phrase)
3. I will leave exactly at five o'clock. (adverb before the time phrase)

4. I will leave at precisely five o'clock. (adverb before the time phrase)
5. I will leave at five o'clock precisely. (adverb after the time phrase)
6. I will leave precisely at five o'clock. (adverb before the time phrase)

7. I will leave at promptly five o'clock. (adverb before the time phrase)
8. I will leave at five o'clock promptly. (adverb after the time phrase)
9. I will leave promptly at five o'clock. (adverb before the time phrase)

10. I will leave at just five o'clock. (adverb before the time phrase)
11. I will leave at five o'clock just. (adverb after the time phrase)
12. I will leave just at five o'clock. (adverb before the time phrase)

In all of the sentences, the adverb is used to indicate precision or punctuality in relation to the specified time. The choice of word order depends on the desired emphasis or style of expression.

Sentences 1-9 and 12 are all correct. Congratulations!!

The adverb "just" is out of place in 10 and 11.