This is a difficult one for me, and it'll take some time for it to sink in. It doesn't follow vowel harmony rules, nor does it change with different conjugations. I'm talking about forming a sentence that starts with "While I was..."
In theory, it sounds simple enough: you add -ken to a verb base, then continue on with the rest of the sentence as normal. But when dealing with different persons, such as "While I... you were", the personal pronouns must always be specified. As an example, "While I was working, they were sleeping." The Turkish sentence would be "Ben çalışıyorken, onlar uyuyordu." (Literally, it means "I, working while, they slept/were sleeping"). The word "çalışıyorken" (working while) - does not change according to the person. It stays the same, whether talking about I, you, we, or whoever else. This is why the personal pronouns need to be specified when talking about two different people doing different things. If talking about the same person for both actions, the personal pronouns don't need to be specified, other than the one person being referenced, either by name or by pronoun.
"tam... -ken" can be used to mean "just as, at the moment that". The example I'll use is "Just as I was leaving the house, it started to rain." The Turkish sentence would be "Ben tam evden çıkarken, yağmur yağmağa başladı." (Literally "I, house from just as I was leaving, rain raining it started.")
There's another use for the "-ken" suffix that uses the future tense. This is probably the most difficult one for me to wrap my head around, but it's really useful, so I need to make a point of learning it. When "-ken" is attached to a future tense, it takes on the meaning of "instead of (do)-ing" something. For example: "Instead of staying in Spain, I went to Turkey." In Turkish, it would be "İspanya'da kalacakken, Türkiye'ye gittim." (Literally, "Spain in, will stay instead of, Turkey to I went.")
So that's what I need to work on the next few days. There are actually more uses for "-ken", but I'm just going to concentrate on these three until I get them down pat.