Can I start a sentence with "For the last ten years...?" and do I need to put a comma after that. For instance, "For the last ten years, I have been doing my PhD". I am grateful for your help.
Yes you can, but a comma is optional.
- For the last ten years, I have been doing my PhD.
"For the last ten years" is a introductory prepositional phrase, and "I have been doing my PhD" is an independent clause. A comma is optional to separate introductory prepositional phrase from the rest of the sentence. In this case, I would use a comma, because the introductory phrase isn't required for the sentence and is there to add detail to it.