Practice with 现在

Could you help me please with these sentences?

你 现在 做什么?
我正在 学习 十五 个汉字.
我 写 这十五个汉字已经三个小时了。但是我学不好它们。

but ：但是
What are you doing?
I am learning 15 Chinese characters. I have been writing them for 3 hours but I still can’t learn them well.
And I don't know how to say "but" in Chinese.

谢谢