Final answer:
The sentence needs an apostrophe in "Kim's" to show possession, proper punctuation around the introductory clause, and could be restructured for tone.
Step-by-step explanation:
The sentence in question, "Kims assistant, usually so reliable, has been late for work three times this week, without any excuse." requires a few grammatical corrections to enhance clarity and professionalism. Firstly, there should be an apostrophe in "Kim's" to denote possession, indicating that the assistant belongs to Kim. Additionally, the sentence lacks punctuation to properly separate the introductory phrase ("usually so reliable"). Including a comma after "Kim's" and before "usually" would help set off this clause. Furthermore, to maintain a friendly and professional tone, the sentence could be restructured to provide some contextual understanding or express concern rather than sounding accusatory.