Learn Turkish: Müteahhit temelin sağlam olduğunu fark etti. - The contractor noticed that the foundation is sturdy.

This sentence uses a factive complement clause for a description. In English, we say 'that the foundation IS sturdy.' Because Turkish needs to turn this 'that' clause into a noun object, and sağlam (sturdy) is an adjective, we use the helper verb olmak (to be). We add the factive suffix -dık to it, making olduk, which acts as the 'is' in 'that it is sturdy.'

Why does temel (foundation) get the genitive suffix -in?

In Turkish complement clauses, the subject of the embedded clause (the thing being described) takes the genitive case, while the verb or helper verb takes a possessive suffix. It literally translates to something like 'the contractor noticed the foundation's being sturdy.' Therefore, temel becomes temelin.

What exactly are the suffixes on olduğunu?

It breaks down as ol (root) + duk (factive suffix, changing to duğ due to consonant mutation) + u (3rd person singular possessive, matching temelin) + n (buffer letter) + u (accusative case). The accusative case is required because this entire clause is the direct object of the verb fark etti (noticed).

Could we use farkına vardı instead of fark etti here?