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.
我已经学了三个小时  but(但是）我还是 学不好它们，而且我不知道中文的“but”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

“but”means“但是”