Learn Turkish: Eskiden ihaleyi kimin kazanacağı şaibeli olmazdı. - In the past, who would win the tender wouldn't be dubious.

In this sentence, the entire clause kimin kazanacağı (who would win) acts as the subject of the main sentence. Because it is the subject, it takes the nominative (zero) case. You typically only add case markers to these -(y)AcAk clauses when they act as the object of the main verb (which would require the accusative case).

Why is the word kim (who) in the genitive case as kimin?

When you create an embedded factive clause using -dIk or -(y)AcAk, the subject of that specific clause must take the genitive case, while the verb takes a possessive suffix. You can think of kimin kazanacağı literally as 'whose future-winning'.

If the clause is the subject, why is ihaleyi in the accusative case?

ihaleyi (the tender) is the specific direct object of the verb kazanmak (to win) inside the embedded clause. Its grammatical relationship is strictly with kazanacağı, so it requires the accusative marker -(y)i regardless of the role the entire clause plays in the broader sentence.

How does the ending on olmazdı change the meaning compared to just saying olmadı?