1. The boys went into the room. Motion is implied here. For instance, with this statement I visualize the movement of the boys from outside the room to inside the room. 2. The boys went inside the room. There is less emphasis on motion (from outside the room to inside the room) as above. It still exists, but it is less. Instead, more emphasis is more placed on the location where the boys ended up - a location within the room. So while the 2 sentences are very similar and are often interchanged, the first places a touch more emphasis on the physical movement, the 2nd on the final location.

"Into" is for motion. "In" is for location. The boys went into the room. They stayed in the room for an hour. "Inside" often refers to the interior of an enclosed area. The boys walked up to the room, then they went inside. "Onto" and "on" are similar to "into" and "in." The cat jumped onto the table. The cat stayed on the table for an hour. English varies by region, by education, by age, and by situation. You will probably get varying opinions.