Learn Turkish: Sabah olunca koridor çok huzurluydu. - When it became morning, the corridor was very peaceful.

It is made of the verb stem ol- (to become, to happen) and the suffix -unca, which means "when" or "as soon as". So sabah olunca literally translates to "when morning happens" or "when morning happened".

How do we know sabah olunca refers to the past if there is no past tense suffix on it?

The suffix -unca doesn't carry a tense of its own. It borrows its time frame from the main verb at the end of the sentence. Since huzurluydu (was peaceful) is in the past tense, we know the "becoming morning" also happened in the past.

How would the meaning change if we used the lesson's focus word, boyunca, instead of olunca?

The word boyunca means "throughout" or "for the duration of." If you changed the beginning to Sabah boyunca, it would mean "The corridor was very peaceful throughout the morning," rather than at the exact time it became morning. Notice that boyunca acts as a postposition directly after the bare noun (sabah), while -unca is a suffix attached to a verb (ol-).

Why isn't there a word for "it" in the Turkish sentence, like in the English translation?