some other ways to express the fact that I have been learning something for a certain period of time but still haven't mastered the ABCs of it?

Solution 1:

I haven't even touched the door to fixing cars.

I've never heard of the expression "to touch the door" although I can guess its meaning because of the context. You could say, "I haven't even started fixing doors." Otherwise, these terms will be more familiar with AmEng speakers—

A rookie (AmEng) "I'm a rookie when it comes to fixing cars."

A novice "I've only just started learning the basics of car mechanics. You could say I'm a novice."

Fixing cars is just like learning Chinese to me.

Not an idiomatic expression, but it's fine if you want to say that learning something is virtually impossible. However, I don't think that's your intention, you have started learning something but you're at the very beginning. Hence, I would suggest the following;

To learn the ropes "I'm far from being a car mechanic, I've only begun learning the ropes."

To get the hang "Learning how to fix cars takes time. I've only started to get the hang of it"

To master "It will be some time before I master the art of car mechanics"

Instead of

I am a green hand at fixing cars

(Which is sort of OK, in the sense that we understand what you're saying.) It's better just to say green.

To be green lacking training; not trained; inexperienced. "I'm still green when it comes to fixing cars"

Solution 2:

(questionable) I am a green hand at fixing cars.

In American usage, this is more likely worded as "I'm a greenhorn at fixing cars."

(grammatically acceptable) Fixing cars is just like learning Chinese to me.

This is grammatically correct and easily understood. But it implies you don't expect to ever learn how to fix cars. A more common idiom is "Fixing cars is all Greek to me."

(wrong) I haven't even touched the door to fixing cars.

This is not at all idiomatic in English. The meaning is clear, but it means you have not spent any time learning about fixing cars, not that you have been learning but still have much more learning to do.

A common way to say what you describe is simply, "I'm just starting to learn about fixing cars."

Solution 3:

"I'm still learning to fix cars."

While masters are masters because they don't stop learning, I think this idiom conveys that you know something, but you are not an expert, and you are continuing to improve.

Solution 4:

You are on the road to being a mechanic.

If you want to emphasize the very early stage of your development, you could say

I am just starting down the road to being a mechanic.