iken, -(y)ken

1. (y)ken is the suffix form of iken, meaning as, (the time) when, while. it doesn't follow the vowel harmony rules, that is, it's always (y)ken no matter what the last vowel of the word is.

başlar ay doğarken saltanatı sultaniyegahın
the majesty of sultaniyegah starts as the moon rises

2. it can be added to nouns or verbs in the 3rd person singular (not all tenses).

2a. since the grammatical person is missing, a noun or pronoun as the subject of the sentence is generally necessary, otherwise the meaning would be ambiguous:

sen o yolları giderken biz dönüyorduk: we were coming back while you were going those roads (*). here sen (=you singular) can't be excluded.

sometimes the meaning is inherently ambiguous and can't be fixed if there's no clue from the context and the sentence should then be restructured or rephrased:

seni arabaya binerken gördüm: i saw you while i was/you were getting on the car. here it's not clear who was getting on the car: i or you? it can only be understood through the physical or verbal context.

2b. if the last letter of the word is a vowel a y is put (for no apparent reason), dropped if it's a consonant:

ben evde-y-ken: while i (am/was) at home.
ben işte-y-ken: when i (am/was) at work.

ben çocukken: when i was a child
kapı açıkken: when the door (is/was) open

as it can be seen, sometimes the tense of the sentence is also ambiguous. it's highly dependent on the physical or verbal context.

2c. tenses that accept the suffix:
remember that (1) the tense of the sentence may be different from that of the verb, and (2) the grammatical person is understood from the context since the verb is always in the 3rd person singular.

i. the present tense
ben doğarken ölmüşüm: i died when i was born (literally, i died when i am born)

ii. the present continuous tense
sen elinin tersiyle ittin ben ağlıyorken: you pushed (me) with the back of your hand while i was crying

iii. the future tense
hani kahramanlar gibi sevecekken seni
masal bitti ...
you know, when i was going to love you like heroes,
the story was over

iv. the past tense with -miş/-mış:
daha çatal kaşık bıçak icat edilmemişken: the time when the fork, spoon and knife weren't yet invented

note:
the past tense with -di/-dı accepts a different suffix -ğinde/ğında or -ği/ğı zaman along with the personal suffix:

when i/you etc came:

geldiğimde = geldiğim zaman (i)
geldiğinde = geldiğin zaman (you, singular)
geldiğinde = geldiği zaman (he/she/it)
geldiğimizde = geldiğimiz zaman (we)
geldiğinizde = geldiğiniz zaman (you, plural)
geldiklerinde = geldikleri zaman (they)

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(*) ie, i'm more experienced or learned than you. here the speaker uses we instead of i to imply and stress his/her superiority in knowledge or experience over the person addressed.