c）means i have learned English.
我过去常学中文。

a) 我在学中文。
I am studying Chinese now.
b) 我学中文了。
(To tell someone that) I study Chinese.
c) 我学了中文。
I have learnt Chinese.
d) I used to study Chinese (but not now).
我学过中文。

a) 我在学中文。
I'm learning Chinese
b) 我学中文了。
I've learnt Chinese(some people may think you are telling them you started to learn Chinese ),you may be still learning it ,or not.
c) 我学了中文。
It's kind of similar to b),but usually you will add soemthing behind it,or you'd better say:我学过中文。
But I think Chinese people would prefer to say:我（曾）学过中文。（I used to learn Chinese ,but i dont know)我一直在学中文。（I've been learning Chinese,and now you are still learning it.)

more example:
我去了北京。（It could means you've been to Beijing ,but now you are back./It could also means that you left for beijing,and you are still staying there)

but if you say:我去过北京。it only means that you have been to beijing before.

I think you are confused about these sentences just like me confused about the perfect tense and past tense in English. The reason may be because we have different attitude about time. There is not direct path between two language about them. You can learn more from the reply of my entry that I asked a question about this.
I think only a) 我在学中文。is idiomatic Chinese.
If you say b) 我学中文了。c) 我学了中文。The meaning is not clear in Chinese.
If you want to say I used to study Chinese (but not now),我以前学中文, 但是现在不学了. I think it will be clearer.
I don't know if you understand me. If you have a question, don't hesitate to send a message to me.

To me 我学中文了 means either:
1) 我要／开始／准备学中文了 (I'm going to / I start now to...)
2) with emphasis on word "学", (but you haven't learnt Chinese!) Yes! Yes! 我已经学了中文了！

wow! there are so many sweet guys have helped you of your question. If you still have some questions on it, I'm glad to help. Enjoy yourself in Chinese study.

For b), I have a comment here.
It is not used alone usually. It often be used as an answer to some questions like
你刚才这两个小时都做什么了？
(What did you do in the past 2 hours? )
And in this case, b) 我(刚才这两个小时)学中文了 means I learned Chinese (in the past 2 hours).
If you use this sentence without such a context, Chinese might feel a little strange though they could understand you.

a) 我在学中文。
b) 我学中文了。
c) 我学了中文。

您好，请您帮助我。 ：）

THIS MEAING IS FOLLOWING ；
I AM LEANING IT
I LEANED IT
I HAVE LEANED IT FOR ME 。
HI， LET ME HELP YOU。

GOOD，頑張って！！！！

thanks everyone for your help!

Good job!