Answer:
I have known Rosa since Primary school when I first met her.
Explanation:
When since is used in a sentence, it is used in relation to time. The verb used in the description is usually in the present perfect or past continuous tense. In the sentence above, the speaker explains that he has known Rosa for a long time.
That time can be dated to primary school when he first met her. Known which is the past participle of know is used when rephrasing the sentence.
