Here's why:
* Direct Objects: They receive the action of the verb. They are directly affected by the verb, so they're placed on the right to show this relationship.
* Predicate Nominatives: These nouns or pronouns rename the subject. They are linked to the subject by a linking verb, so they're placed on the right side, parallel to the subject.
* Predicate Adjectives: These adjectives describe the subject. They are also linked to the subject by a linking verb, so they are placed on the right side, parallel to the subject.
Example:
"The dog became a friend."
* The dog: Subject (on the left side of the main line)
* became: Linking verb (on the main line)
* friend: Predicate nominative (on the right side of the main line)
Diagram:
```
_________________
| |
| The dog |
| |
| became |
| |
|_________________|
|
|
friend
```
Let me know if you'd like me to diagram any other sentences for you!