1. The problem is too difficult/hard for me to solve.2. The problem is so difficult/hard that I can't solve it.3. The problem is very difficult/hard so that I can't solve it.4. The problem isn't easy enough for me to solve.5. This is such a difficult/hard that I can't solve it.6. This is so difficult/hard a problem that I can't solve it.注:/ 表示可以任意选用其一;还可以把solve改写成work out,solve it改写成work it out.