Breakdown of Onların yalan söylediği malum, yeni bir plan yapmalıyız.
bir
a / an
yeni
new
onlar
they
söylemek
to say, to sing
Questions & Answers about Onların yalan söylediği malum, yeni bir plan yapmalıyız.
Why doesn't söylediği have an accusative or other case marker here?
Because it is the subject of the first clause ('That they lied'), not the object. When a factive clause formed with -dIk acts as the subject of a sentence, it takes zero case endings. The clause literally acts like a noun phrase meaning 'Their past lying', which functions as the subject of the adjective malum.
Why does onların (plural 'their') pair with söylediği (singular 'his/her/its saying')? Shouldn't it be söyledikleri?
You can definitely say söyledikleri, but it isn't strictly required. When the subject of a clause is a plural pronoun like onların, Turkish speakers often drop the redundant plural marker on the verbal noun. Using the singular possessive -i here is very common and sounds perfectly natural.
Where is the main verb in the first clause, Onların yalan söylediği malum?
If we wanted to say 'That they will lie is known', how would the clause change?