Learn Farsi: او دیروز را در تهران گذراند و برگشت. - He spent yesterday in Tehran and returned.

In the simple past tense, the third-person singular (meaning "he", "she", or "it") is the easiest form to make! You simply use the past stem of the verb without adding any extra personal endings. The past stem always ends in -d or -t, which is exactly what we see in گذراند (spent) and برگشت (returned).

Why is the object marker را attached to دیروز (yesterday)?

In English, we usually use "yesterday" as a time word to say when something happened. But in this sentence, it is the direct object of the verb. He didn't just do an activity yesterday; he actively "spent yesterday". Because "yesterday" is the specific thing being spent, it requires the specific direct object marker را.

How do we know who returned, since there is no pronoun after و (and)?

Farsi frequently drops subject pronouns when the person doing the action hasn't changed. Because the sentence starts with او (he) and the second verb برگشت is also in the third-person singular form, it is perfectly clear that he is still the one returning. You don't need to repeat the pronoun.

How do I find the past stems used here if I only know the dictionary forms?