Learn Russian: С тех пор как я уехал, я расширил свой кругозор. - Since I left, I have expanded my outlook.

The phrase с тех пор как literally means "from those times when." While the simple preposition с means "since" before a noun (like с мая - "since May"), you must use the full phrase с тех пор как to introduce a temporal subordinate clause that has its own subject and verb, like я уехал ("I left").

Why is расширил in the past tense when the English translation uses the present perfect ("have expanded")?

Russian doesn't have a distinct present perfect tense. Because the act of expanding your outlook is viewed as a successfully completed action, Russian simply uses the past tense of the perfective verb (расширить). The combination of с тех пор как and a past perfective verb naturally translates to the English present perfect, linking a past event to a current result.

Could I use мой instead of свой here since the subject of the clause is я?

You could technically use мой, but свой is much more natural and preferred. As a rule, whenever a possessive pronoun refers back to the grammatical subject of the exact same clause (in this case, "I expanded my own outlook"), Russian strongly favors the reflexive possessive свой over personal possessives.

The word кругозор translates to "outlook", but what is its literal meaning?