"wa" vs "o" usage

I've worked on Modules 1.0-1.8 and now my brain has frozen. Things have been gradually been making some sense to me, but, using both RJ and outside sources, I can't seem to comprehend this...

In module 1.8, phrases are given such as, "Sushi wa suki desu ka?" and, "O-sushi o tabemasu."

What is the point of choosing "wa" over "o"?

I get that "o" is the particle that defines the word before it as the object of the verb that follows. I also get that "wa" defines the topic, creates context, and may be translated like "speaking of ______." But my neural pathways seem to have suddenly broken down when it comes to the reasoning as to why, at least with some of these examples, I would opt for "wa" rather than "o".

For example, I choose the "o" in the latter statement of "O-sushi o tabemasu" because sushi is the object of eating, but, in the former example, isn't sushi the object that is being liked?

In a similar way, consider the statements from Module 1.4: "Anata wa nihongo ga hanasemasu ka?" and, "Dure gurai nihongo o benkyo shite imasu ka?"

"o" is being used in the latter example because "nihongo" is the object of studying. How come, then, in the former example, "ga" is used rather than "o" which would show that "nihongo" is the object of speaking?

In the former example, should "o" never be there at all, or does the "ga" for some reason take priority?

Thank you much!

Trying giving these links a read:

https://8020japanese.com/wa-vs-ga/

https://www.wasabi-jpn.com/japanese-grammar/japanese-particles/japanese-particle-table/
(this link requires some knowledge of hiragana)

Thanks for the links. I can't read hiragana yet, but I've read the 80/20 article twice--I guess I'll be reading it again. I actually got Richard Webb's book, 80/20 Japanese, in the mail today, so I'll have to take a look in there.

How about this link?
http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/

This Rocket thread might also be helpful though hiragana is used throughout:
https://members.rocketlanguages.com/members/forum/japanese-grammar/ni-o-particles

This Rocket thread mentions ga vs wa using romaji:
https://members.rocketlanguages.com/members/forum/japanese-grammar/ga-vs-wa

I'd ignore the lesson order Rocket Japanese gives for Writing Lessons, and just learn hiragana and katakana ASAP.​ If you learn a line (a, i, u, e, o | ka, ki, ku, ke, ko | etc.) every day or every other day you can learn to read kana in under a month.  That will open up resources and native materials to you.

Thank you again. That NihonShock article was a big help. I also did some reading and exercises from 80/20 Japanese that really helped. I still need a lot of work, but I've made some headway regarding "wa," "ga," and "o".

...My, oh, my... I just sat here for the last 10 minutes writing more related to still having a major lack of understanding as to the usage difference of "o" and "ga" in statements from Module 1.9 such as "Nihongo o hanashimasu," and, "Nihongo ga hanasemasu."

As I wrote my question, I looked at the Nihon Shock cheat sheet (http://cheatsheets.nihonshock.com/sheets/basic-japanese/) I downloaded from a link in the article you sent me. It says:

"ga" - Marks objects of potential-form verbs ("can"/"able to")

That's what I've been looking for! I had no idea "ga" could be used as an object marker. It was so obvious, but for some reason I kept relating it more to "wa" and was thinking that, even though it seemed to be marking an object, perhaps something had fundamentally changed gramatically that shifted the object to some other part of speech. 

Arigato gozaimasu!!!  :-)