Learn Turkish: Denizaltı yavaş ilerlediği sürece bir tehlike yok. - As long as the submarine advances slowly, there is no danger.

It uses the structure you are currently learning. We take the verb root ilerle- (to advance) and add the suffix -dik, which softens to -diğ- because it is followed by a vowel. Then we add the third-person possessive suffix -i to match the submarine. Combined with the word sürece, it creates the meaning as long as it advances.

Why isn't the subject denizaltı marked with the genitive case like denizaltının here?

Usually, the subject of a -dik clause takes the genitive case, just like in benim geldiğim (my coming). However, with the sürece structure, third-person subjects often stay in their plain dictionary form. So it is much more natural to say denizaltı ilerlediği sürece rather than adding the genitive marker.

Is denizaltı a compound word?

Yes, it is made of deniz (sea) and alt (bottom or under) plus the compound linking vowel -ı. It literally means underside of the sea but is the standard word for submarine. Since the linking vowel is already built into the word, it just acts as a single root noun in this sentence.

Why do we use yavaş instead of an adverb form like yavaşça for slowly?