I have an invention whose principle is the same as others' patent, but its application is different. Can I patent this?

First, carefully study other people's patents and see what is the difference between the patent you want to apply for and others' patents, and whether these differences have technical effects. If so, there is no problem in applying for an invention patent.

Can the technical scheme of patent A be applied to B? This is a modified invention. If the technical scheme of patent A is applied to B, a patent can be obtained. If it does not produce special technical effects, you can also apply for a patent, and the possibility of approval is very small.