Learn Finnish: Yksinkertainen suggestio saa ihmiskehon reagoimaan nopeasti. - A simple suggestion gets the human body to react quickly.

The verb saada changes its meaning to to make someone do something or to get someone to do something when it is paired with an object and the third infinitive illative (-maan or -mään). In this sentence, saa ihmiskehon reagoimaan literally means gets the human body into reacting.

Why is the object ihmiskehon in the genitive-looking accusative case instead of the partitive?

The action is treated as a complete whole: the suggestion successfully gets the body to react. When a causation structure with saada expresses a completed or successful result, its object takes the total object case (ending in -n here). If the attempt were unsuccessful or ongoing, it would use the partitive case.

How is the form reagoimaan built from the dictionary verb reagoida?

Reagoida (to react) is a type 2 verb. To form the third infinitive, you take the strong verb stem by dropping the -da to get reagoi-, and then simply attach the -maan ending. It requires this specific ending because the main verb saada demands that the action being caused takes the -maan/-mään form.

Could this sentence be misunderstood as the human body gets a simple suggestion to react quickly?