Those English verb forms have different ,what are called, aspects. The verb aspects vary from language to language. Finnish aspects differ, and to my knowledge are not taken up on this basic course at all.
Having said that grillaa can mean "gills", "is grilling" and even "will grill" depending on the context.
I would say that the one with the partitive object
- Kanadalainen grillaa tomaatteja.
is the best choice for a generalised action statement. This is actually a case where many native speakers make an error. During a summer there are sales booths at lay-bys where blueberries, peas and so on, but above all strawberries are sold. Quite often you see a sign
- Mansikkaa 300 m
indicating that there is a strawberry sales booth at the next lay-by (300 m or whatever the distance is). If taken literally there is a single strawberry at sale :-(
Take a piece of tinfoil, make it to a bowl-shape form and put some oil and spices, e.g. olive oil and black pepper (keep it simple), onto it. Fold the tomatoes into the foil and make it a small bag. The tomatoes will become ready quickly, so put the bag onto the grill ,say, five minutes before the sausages become ready. Open the bag so that the steam comes out and the tomatoes get barbequed instead of boiled in their own juice. Serve hot.