This is the 598th most frequent Swedish word.
personen
"The person."
In this sentence, 'personen' is used as a noun referring to a singular individual, with a definite article implied by the suffix '-en'.
Personen står vid dörren.
The person is standing by the door.
Here, 'personen' serves as the subject being thought about and is defined by the relative clause 'som hjälpte mig igår'.
Jag tänkte på personen som hjälpte mig igår.
I thought about the person who helped me yesterday.
In this instance, 'personen' is used in a question as a direct reference to a specific individual defined by the context.