Learn Finnish: Mies heittää rikkaruohot naapuria kohti. - The man throws the weeds toward the neighbor.

To say he throws his own weeds, you would add the third-person possessive suffix -nsa/-nsä to the object: Mies heittää rikkaruohonsa naapuria kohti. Because the owner (the man) is also the subject of the sentence, we omit the genitive pronoun hänen (his). If you included hänen here, it would actually mean he is throwing somebody else's weeds! (Note: the plural -t ending disappears when you add a possessive suffix).

Why is rikkaruohot in the t-plural instead of the partitive plural (rikkaruohoja)?

The t-plural object here acts as a "total object." It tells us that the man is throwing all of those specific weeds, and the action is viewed as a completed whole. If we used the partitive rikkaruohoja, it would mean he is throwing an unspecified amount of weeds (some weeds), or that the throwing is an ongoing, continuous process.

Why is the word for neighbor in the partitive case (naapuria)?

It is governed by the postposition kohti (toward). While most Finnish postpositions require the word coming before them to be in the genitive case, kohti is a common exception that always requires the partitive case.

What are the root words that make up the compound word rikkaruohot?

It is made of rikka (a speck of dirt, dust, or debris) and ruoho (grass or herb). Literally "trash grass", this is the Finnish word for a weed.