Learn Turkish: Dizim çok kanıyordu, az kalsın bayılıyordum. - My knee was bleeding a lot, I almost fainted.

When combined with az kalsın (almost), the Past Continuous tense (-(i)yordu) describes an action that was on the verge of happening but was averted. If you used the Simple Past (bayıldım), it would mean you actually fainted. The past continuous gives the feeling of 'I was right in the process of fainting, but didn't.'

The verb for 'to bleed' is kanamak. Why is it spelled kanıyordu instead of kanayıyordu?

This happens because of vowel narrowing. When you add the continuous suffix -yor to a verb ending in the broad vowels a or e, that vowel narrows to ı, i, u, or ü. Here, the a in kana- narrows to ı, creating kanıyordu.

Where does the phrase az kalsın come from grammatically?

It is a fixed idiom, but literally translates to 'let little remain.' It combines az (little) with kalsın, which is the third-person imperative form of kalmak (to stay or remain). It paints a picture that very little time or distance stood between you and the averted action.

Could I use neredeyse instead of az kalsın in this sentence?