Practice with 现在

Could you help me please with these sentences?

你 现在在 做什么?
我现在 在 学习 十五个 汉字.
我 已经花了三个小时的时间去写它们了，但是仍然学不好，并且我不知道“but”(但是)用汉语怎么说。
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.

hi，“but”的中文表达是：但是