Learn Russian: Наводнение разрушило деревню, так что нам следует помочь жителям. - The flood destroyed the village, so we ought to help the residents.

In this sentence, так что acts as a single conjunction meaning "so" or "with the result that." When it introduces a consequence, the comma goes before the whole phrase. If you split it with a comma (так, что), it changes the grammar and means "in such a way that."

Could we use поэтому instead of так что here?

Yes, they are very similar here! However, так что creates a subordinate clause closely tied to the main event (the flood), while поэтому (therefore) usually acts as an adverb starting a new independent clause. так что feels a bit more like a natural "so" connecting two thoughts.

Why is it нам следует instead of using the nominative мы?

When следовать means "ought to" or "should," it acts as an impersonal verb. The person who should do the action must be in the dative case (нам). If you used the nominative and conjugated the verb (мы следуем), it would mean "we are following" a physical path or a person.

Why is жителям in the dative case?

The verb помочь (and its imperfective pair помогать) always requires its target object to be in the dative case. Here, it is the dative plural form of житель (resident).