"Hey, Joe. Good to see ya'. I'm going to say a sentence with a direct object and you repeat it back to me using direct object pronouns."
Yet that's what you're being asked to do when you do a classic grammar activity such as:
1. Leo el libro. >> ____ leo. [and you write: Lo leo.]
In real life, you'd more likely use it this way:
-¿Leíste el libro?
-Sí, lo leí.
But even that is still a bit artificial since most people would just answer with a simple Sí or No.
You would really use the pronouns to avoid repetition when you're adding new information. Like this:
-¿Leíste el libro?
-Sí. Se lo presté a mi hermana. [Yes. I loaned it to my sister.]
So once you clear up the fact that most of what you do know about direct and indirect object pronouns probably wouldn't be very helpful in any real world situation, here are a few pointers that should help to make more sense of it.
What's a direct object anyway?: start with the verb and ask yourself "who? or what?" The answer is the direct object.
What's an indirect object?: start with the verb and ask yourself "to whom? or for whom?" The answer is the indirect object*.
To use direct and indirect object pronouns in Spanish, follow these four steps to clarify it all:
1) ANSWER the question (get your subject & verb straight)
2) FIND the direct and indirect objects
3) REPLACE the direct and indirect objects with the appropriate pronouns (me, te, lo/la, nos, os, los/las; me, te, le (se), nos, os, les (se)**)
4) POSITION the pronouns (indirect, then direct) before the conjugated verb (or after & attached to an infinitive, gerund or command).
Try it with our original question:
¿Leíste el libro?
1. Sí, leí el libro. (You have to answer the "tú" question with "yo")
2. read what? -el libro
3. Replace "el libro" with the pronoun "lo"
4. Position before verb: Sí, lo leí.
*Certain verbs will take indirect objects, like dar, decir, regalar, enviar/mandar, hacer, pedir You give something to someone, say something to someone, send something to someone, give something to someone, etc.
** “Can’t have two ‘l’ words in a row” so “le” becomes “se” before lo, la, los, las.