@cicerone imposter,
So that cdog67 doesn't think that word order was the problem, I'll add that the following would be perfectly fine:
After he finishes high school, Tom is going to university.
At 3:00, Sarah is going to swim with her friend, Marsha.
In the first sentence, the verb tense of "finish" was incorrect. In the second, the word "play" is not appropriate since swim is itself used as a verb (unlike if you used "basketball", for example) and "Marsha" generally takes a comma before because it is an appositive. An appositive is a noun or noun phrase that renames another noun right beside it, as if you were adding a detail that is secondary.