what is the difference between a subjective complement and a direct object?

It is also the direct object.For instance if; Bobby kicked the blue ball sideways to Joe.

Then Joe would be an indirect object but the ball remains the direct.

Types of object

Objects fall into three classes: direct objects, prepositional objects, and non-prepositional indirect objects. An indirect object is the recipient of the direct object, or an otherwise affected participant in the event. For example, if three sentences are considered:

* In “We threw knives”, knives is the direct object of the verb threw.

* In “We listened to the radio”, the radio is the object of the preposition to, and the prepositional object of the verb listened.

* In “They sent him a postcard”, him is the (non-prepositional) indirect object of the verb sent (which uses a double-object construction).

In English the type of object is indicated strictly by word order.

http://en.wikipedia.org/wiki/Direct_object

My words::A lot of English can be figured out by looking at the sentence to see what the individual words are doing.Each sentence will be different of course and the trick is to see what all of the words do.

I have heavily edited this Wikipedia entry.Look where it says that in English the type of object is indicated strictly by word order.You can figure it out if you search hard enough.It only makes sense after long study.